From the Department of Chemical Engineering, North
Carolina State University, Raleigh, North Carolina 27695 and
§ SAS Institute Inc., Cary, North Carolina 27513
Received for publication, November 18, 2002
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ABSTRACT |
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The hyperthermophilic bacterium
Thermotoga maritima MSB8 was grown on a variety of
carbohydrates to determine the influence of carbon and energy source on
differential gene expression. Despite the fact that T. maritima has been phylogenetically characterized as a primitive
microorganism from an evolutionary perspective, results here
suggest that it has versatile and discriminating mechanisms for
regulating and effecting complex carbohydrate utilization. Growth of
T. maritima on monosaccharides was found to be slower than
growth on polysaccharides, although growth to cell densities of
108 to 109 cells/ml was observed on all
carbohydrates tested. Differential expression of genes encoding
carbohydrate-active proteins encoded in the T. maritima
genome was followed using a targeted cDNA microarray in conjunction
with mixed model statistical analysis. Coordinated regulation of genes
responding to specific carbohydrates was noted. Although glucose
generally repressed expression of all glycoside hydrolase genes, other
sugars induced or repressed these genes to varying extents. Expression
profiles of most endo-acting glycoside hydrolase genes correlated well
with their reported biochemical properties, although exo-acting
glycoside hydrolase genes displayed less specific expression patterns.
Genes encoding selected putative ABC sugar transporters were found to
respond to specific carbohydrates, and in some cases putative
oligopeptide transporter genes were also found to respond to specific
sugar substrates. Several genes encoding putative transcriptional
regulators were expressed during growth on specific sugars, thus
suggesting functional assignments. The transcriptional response of
T. maritima to specific carbohydrate growth substrates
indicated that sugar backbone- and linkage-specific regulatory networks
are operational in this organism during the uptake and utilization of
carbohydrate substrates. Furthermore, the wide ranging collection of
such networks in T. maritima suggests that this organism is
capable of adapting to a variety of growth environments containing
carbohydrate growth substrates.
Saccharolytic microorganisms employ a range of proteins to
hydrolyze, transport, and utilize complex carbohydrates that serve as
carbon and energy sources (1). In some cases, these proteins are very
specific to particular carbohydrates, whereas in other situations they
mediate the processing of a broader range of glycosides. For simple
sugars, such as glucose, binding and transport proteins alone mediate
substrate entry into specific intracellular anabolic and catabolic
pathways (2). However, for complex carbohydrates, a series of glycoside
hydrolases must first process the polysaccharide so that its backbone
and side chain glycosidic linkages are hydrolyzed to the extent needed
for binding, transport, and intracellular utilization. How specific
organisms develop the capacity to utilize complex carbohydrates is not
known, but this probably involves evolutionary pressures in addition to
acquisition of this genetic potential through horizontal gene transfer
events. In any case, a microorganism's capacity to utilize
carbohydrates presumably reflects the availability of such substrates
in its habitat. Therefore, insights into the repertoire of
carbohydrate-active proteins in a given organism and how the expression
of these proteins is regulated would reveal much about particular
metabolic features in addition to how it interacts within a given ecosystem.
Thermotoga maritima is an obligately anaerobic,
heterotrophic, hyperthermophilic bacterium originally isolated from
geothermal features associated with Vulcano Island, Italy (3). Its
capacity to utilize a wide range of simple and complex carbohydrates
was confirmed by the inventory of glycoside hydrolases encoded in its
genome (4). In fact, the T. maritima genome, despite its relatively small size, encodes the largest number of glycoside hydrolases of any bacterial or archaeal genome sequenced to date (see
Fig. 1). From growth experiments and
characterization of specific glycoside hydrolases (5), T. maritima is known to metabolize both polysaccharides and simple
sugars, including carboxymethylcellulose, barley glucan, starch,
galactomannan (5), xylan (6),
pectin,1 mannose, xylose, and
glucose (2). In some cases, the proteins involved in the processing,
transport, and utilization of these glycosides can be inferred from
their apparent organization into operons in the T. maritima
genome sequence (4), whereas in other cases such classification is not
clear. Regulation of genes encoding specific carbohydrate-active
proteins in T. maritima has only been studied to a limited
extent thus far (5, 7), and the coordinated regulation of related genes
involved in polysaccharide utilization has not been examined.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSION
REFERENCES
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Fig. 1.
Carbohydrate active enzymes in completed
genome sequences of thermophilic and hyperthermophilic archaea and
bacteria. B. subtilis and E. coli are shown
for comparison purposes. Data available on the World Wide Web at
afmb.cnrs-mrs.fr/CAZY.
Here, a targeted cDNA microarray, based on
carbohydrate-active proteins from T. maritima, was used in
conjunction with mixed model analysis (8, 9) to explore issues related
to saccharide utilization by this organism. Despite the fact that
T. maritima has been phylogenetically characterized as a
primitive microorganism from an evolutionary perspective (10), results
here support that it has versatile and discriminating mechanisms for
regulating and effecting complex carbohydrate utilization. The relative
importance of evolutionary processes and horizontal gene transfer (4) in developing its carbohydrate utilization capacity is not known, but
T. maritima's ability to respond to various
substrates in its growth environment underlies its ubiquity in global
geothermal settings (11).
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EXPERIMENTAL PROCEDURES |
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Construction of the Targeted cDNA Microarray-- Open reading frames (total of 269) of known and putative genes related to sugar processing and other related metabolic functions were identified through BLAST (12) comparisons of protein sequences from the T. maritima MSB8 genome available on the World Wide Web at www.tigr.org/ tigrscripts/CMR2/GenomePage3.spl?database=btm. DNA primers were designed with similar annealing temperatures and minimal hairpin formation using Vector NTI 7.0 (Informax, Bethesda, MD). The selected probes were PCR-amplified in a PTC-100 Thermocycler (MJ Research, Inc., Waltham, MA) using Taq polymerase (Roche Molecular Biochemicals) and T. maritima genomic DNA, isolated as described previously (5). The integrity and concentration of the PCR products were verified on 1% agarose gels. PCR products were purified to 100 ng/µl using 96-well QIAquick PCR purification kits (Qiagen, Valencia, CA), resuspended in 50% Me2SO, and printed onto CMT-GAPS aminosilane-coated microscope slides (Corning Glass) using a 417 Arrayer (Affymetrix, Santa Clara, CA) in the North Carolina State University Genome Research Laboratory (Raleigh, NC). Eight replicates of each gene fragment were printed onto each slide. The DNA was then attached to the slides by UV cross-linking using a GS GeneLinker UV Chamber (Bio-Rad) set at 250 mJ and baked at 75 °C for 2 h.
Growth of Thermotoga maritima and RNA Isolation--
Growth of
T. maritima MSB8 cultures in artificial sea water was
followed using optical density measurements and epifluorescence microscopic cell density enumeration, as described previously (5).
Growth substrates glucose, mannose, xylose, -xylan (birchwood), laminarin (Laminaria digitata), and starch (potato) were
obtained from Sigma. Galactomannan (carob), glucomannan (konjac),
carboxymethylcellulose, and
-glucan (barley) were obtained from
Megazyme (Wicklow, Ireland). Growth substrates were prepared as
described previously (5) and included in the medium at a final
concentration of 0.25% (w/v). Substrate purities as provided by the
manufacturers varied from 95 to 99%. To ensure minimum carryover
between substrates, cells were grown for at least 10 passes on each
carbon source using a 0.5% (v/v) starting innoculum before obtaining
the growth curves. Specific growth rates on mono- and polysaccharide
substrates were determined from the slopes of semilog plots of
exponential cell growth versus time. Isolation of total RNA
from T. maritima was performed on cells that were grown
until early- to mid-exponential phase on the various growth substrates,
as described in detail previously (5).
Labeling and Hybridization-- First-strand cDNA was prepared from T. maritima total RNA using Stratascript (Stratagene, La Jolla, CA) and random hexamer primers (Invitrogen) by the incorporation of 5-[3-aminoallyl]-2'-deoxyuridine-5'-triphosphate (Sigma) as described elsewhere (13). The slides were scanned using a Scanarray 4000 scanner (GSI Lumonics and Billerica) in the North Carolina State University Genome Research Laboratory. Signal intensity data were obtained using Quantarray (GSI Lumonics).
Experimental Design and Data Analysis-- A loop design was constructed (see Fig. 2) to ensure reciprocal labeling for all 10 different experimental conditions. Replication of treatments, arrays, dyes, and cDNA spots allowed the use of analysis of variance (ANOVA)2 models for data analysis. ANOVAs are especially appropriate for loop designs in which a large number of conditions are compared with one another, eliminating uninteresting reference samples and allowing for the collection of more information on experimental conditions (14). Mixed ANOVA models, in which some effects are considered fixed and others are considered random, have been used to re-examine published microarray data sets (9) and examine the effects of sex, genotype, and age on transcription in Drosophila melanogaster (8).
Using existing SAS procedures and customized Perl code, an
automated data import system was developed to merge Quantarray intensity measurements, coordinate files generated by the array printer, and corresponding T. maritima locus numbers in a
SAS data set (SAS Institute, Cary, NC). The data import system was verified through independent calculations in Excel (Microsoft, Seattle,
WA). A linear normalization ANOVA model (9) of log base 2 intensities
was used to estimate global variation in the form of fixed (dye,
treatment) and random (array, pin within array, pin spot within array)
effects and random error using the following model:
log2(yijklmn) = m + Dj + Tk + Ai + Ai(P1) + Ai(SmPl) + ijklmn. The estimated effects calculated from this model
were used to predict an expected intensity for each value, and then a
residual was calculated as the difference between a replicate's
observed and predicted intensity and then used as data to capture
variation attributable to gene-specific effects after accounting for
global variation. Gene-specific ANOVA models were then used to
partition variation into gene-specific treatment effects, dye effects,
and the same hierarchy of random effects described previously.
Specifically, the model rijklmn = m + Di + Tk + Ai + Ai(P1) + Ai(Sm P1) +
ijklmn was fit separately to the residuals for each gene,
and the resulting parameter estimates and S.E. values were then used
for statistical inference.
Volcano plots were used to visualize interesting contrasts or
comparisons between two treatments or two groups of treatments (9). A
Bonferroni correction was utilized to adjust for the expected increase
in false positives due to multiple comparisons (9). Genes meeting the
Bonferroni significance criteria were selected for further study,
ensuring that genes with inconsistent fold changes would be eliminated
from further analysis. Two complementary approaches were utilized to
cluster data from T. maritima growth on 10 saccharides. To
visualize the relative expression levels of all genes within
a treatment, hierarchical clustering was performed on least squares
means calculated from the linear models for each sugar (Fig. 3). To
visualize the expression pattern of each single gene across
treatments, the least squares mean estimates were standardized using
the mean and S.D. of the 10 least squares means estimates for a given
gene. Each of the 10 least squares means estimates were standardized
accordingly with the formula Yi = (Xi µ)/
, where Yi = the
standardized least squares means variable, µ =
Xi/n, and
= (
(Xi
µ)2)1/2. The
standardized variable was then utilized for clustering (Fig. 3). For
complete information on signal intensity, significance of expression
changes, -fold changes, pairwise volcano plots, and hierarchical
clustering for all of the genes included on the array, see the
Supplemental Material.
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RESULTS AND DISCUSSION |
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Design of Targeted cDNA Microarray-- A targeted cDNA microarray for T. maritima was constructed that included 269 known and putative genes or about 15% of the total open reading frames in the T. maritima genome. This included the known set of genes related to glycoside utilization and modification (65 genes), proteolysis (40 genes), stress response, and proteolytic fermentation. Genes related to sugar transport (21 genes) or transcriptional regulation (69 genes) and 66 other genes of interest were also included.
Genes apparently related to glycoside utilization and modification in T. maritima include 41 glycoside hydrolases, 17 glycosyl transferases, 6 carbohydrate esters, and 1 polysaccharide lyase. The corresponding encoded proteins have been classified into several families, based on amino acid sequence homology (15) (available on the World Wide Web at afmb.cnrs-mrs.fr/CAZY). There are over 130 T. maritima proteins with sufficient BLAST homology to be classified into transcriptional regulatory or signal transduction COG categories (16). These regulatory proteins have been assigned to families based on sequence homology; however, different proteins in the same families may have different DNA and substrate-binding specificities (17). Also, proteins placed in different families may share the same name because of their regulon composition, as in the case of the Escherichia coli and Bacillus subtilis xylR protein (18, 19). Of the 69 transcription/transduction genes on the array, six share similarity with the ROK (receptor, open reading frame, kinase) family of transcriptional regulators, which include glucokinases, B. subtilis XylR, and E. coli NagC (COG1940) (20). Six members of the PurR/LacI superfamily (COG1609) were included (21) along with the T. maritima IclR transcriptional regulator, whose structure was recently solved (22). Several pairs of sensor histidine kinases and response regulators of putative two-component regulatory systems were included, as were regulators from the MarR (23), AraC (24), TroR (25), LytR (26), ArsR (27), and CspC (28) families. The T. maritima genome contains ~120 genes involved in oligopeptide/sugar transport. In the targeted microarray used here, 21 genes related to sugar transport were included on the basis of their proximity to the genes involved in glycoside utilization.
This targeted microarray was used to examine the differential response
of T. maritima grown on a range of mono- and polysaccharides at its optimal growth temperature of 80 °C. Growth conditions were
analyzed based on an incomplete loop design (Fig.
2). Treatments in the loop design were
balanced with respect to dyes so that treatment effects were not
confounded with dye effects.
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Growth of T. maritima on Monosaccharides and
Polysaccharides--
T. maritima cultures were grown on a
variety of saccharides, including the monosaccharides glucose, mannose,
and xylose. The polysaccharides investigated differed in backbone sugar
type (glucose, mannose, and xylose), backbone linkage type (-1,3;
-1,4; or
-1,4), and side chain residue type (galactose,
glucuronic acid, or glucose) (see Table
I). Included in these were a mixed
backbone (konjac glucomannan: glucose/mannose) and a mixed
linkage (barley glucan:
-1,4/1,3) polysaccharide. Final cell
densities were in the range of 108 to 109
cells/ml in all cases. Doubling times (min) for galactomannan (carob),
-glucan (barley), laminarin (L. digitata),
-xylan
(birchwood), starch (potato), glucomannan (konjac), and
carboxymethylcellulose were estimated to be 85, 72, 143, 61, 117, 74, and 78, respectively. On the monosaccharides, the doubling times (min)
were 162, 253, and 188, for glucose, mannose, and xylose, respectively.
Under identical conditions, the average doubling time for growth on monosaccharides (201 min) was observed to be substantially higher than
that on the corresponding polysaccharide substrates (90 min).
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Substrate-dependent Differential Expression of
Genes--
Two hierarchical clusters are shown in Fig.
3 to summarize the expression patterns of
269 T. maritima genes during growth on 10 saccharides. The
first cluster is based on least squares means and compares the
normalized expression levels of all genes within each treatment
condition. The second cluster is based on standardized least squares
means for a single gene across all 10 treatments to show the effect of
different treatments on the relative expression of a particular gene.
The hierarchical clustering based on standardized least squares means
revealed many cases of apparent co-regulation of genes within potential
operons (29). Several sets of spatially distant gene strings were
observed to cluster with similar expression profiles, suggesting the
presence of regulons in the T. maritima genome.
Representative clusters are displayed in Fig.
4.
Overall expression levels of a number of genes remained consistently
high or low regardless of the growth condition. These included
constitutively expressed genes like TM0017 (pyruvate ferredoxin
oxidoreductase) and TM0688 (glyceraldehyde-3-phosphate dehydrogenase)
(30) as well as genes related to proteolytic activity. Both sets of
genes with the corresponding known or putative functions are displayed
in Fig. 5. Individual genes with high overall expression levels on only a single carbon source are indicated in Table II. Least squares means for all
genes included in this study for all growth conditions are shown in
Supplemental Table IV, along with the corresponding standardized values
in Supplemental Table V. Below, gene regulation patterns within each
functional category are examined for each monosaccharide and
corresponding polysaccharide growth substrate.
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Glucose and Glucan Polysaccharides--
Backbone- and
linkage-specific gene regulation was observed in the case of
endoglycoside hydrolase genes for growth on - and
-specific
glucans. Growth on carboxymethylcellulose (CMC) (see cluster 4.1), a
-1,4-linked glucose polymer, induced genes encoding extracellular
endoglucanases TM1525 (cel12B) and TM0305 (cel74), as well as the intracellular endoglucanase TM1524
(cel12A) and the intracellular cellobiosyl phosphorylase,
TM1848. Examination of cluster I (Fig. 3) reveals that expression
levels of cel74 were substantially lower than those of
cel12A on glucan polysaccharides. Although the presence of a
-1,4-glucosidase gene (bglA) (accession number CAA52276)
in T. maritima MSB8 has been reported (31), the
corresponding protein sequence does not show homology to deduced
sequences identified in the T. maritima MSB8 genome (4).
Recently, bglA was reported to be present in T. maritima RQ2 (11).
A switch in sugar backbone linkage from -1,4 to
-1,3 (cluster
4.2) resulted in specific expression of genes encoding a laminarinase, TM0024 (lam16), as well as the corresponding exoglycosidase,
TM0025 (cel3), with comparable overall expression levels.
The ROK family protein TM0032 is located upstream of lam16
and cel3 and displays a similar expression pattern. These
expression patterns suggest specific regulatory networks for each
glucan that differ only by backbone linkage. The mixed linkage
-glucan from barley induced genes encoding
-1,4- as well as
-1,3-glucanases in addition to a related glycosyl transferase,
cellobiosyl phosphorylase; however, overall expression levels of
lam16 were lower on this substrate than on laminarin.
Starch, an -glucan, induced expression of genes encoding the
extracellular endo-acting
-amylase TM1840 (amy13A), a
debranching pullulanase, TM1845 (pul13), and the
intracellular exo-acting
-glucosidase, TM1068 (amy4A)
(cluster 4.3). Expression of the
-glucuronidase TM0434 (32),
previously classified as a putative
-glucosidase, also increased on
starch. The cyclomaltodextrinase TM1835 was significantly induced in
the presence of starch. This intracellular protein has been recently
shown to have exoglycosidase activity at the reducing end in addition
to maltodextrin decycling activity (33). The genes encoding a
4-
-glucanotransferase (TM0364) and maltodextrin glycosyl transferase
(TM0767) were also up-regulated during growth on starch, the latter
also being expressed during growth on glucose and laminarin.
Interestingly, other genes putatively assigned for starch degradation,
including an
-amylase, TM1650 (amy13B), as well as the
putative
-glucosidases, TM0752 and TM1834 (amy4AC),
remained unaffected by this substrate, suggesting a different
intracellular role perhaps related to utilization of storage
polysaccharides like glycogen.
The relative expression of the AraC family regulator TM1005 increased specifically on starch. AraC proteins have been shown to act as transcriptional activators for a variety of substrates (24). A related MarR regulator TM0710 showed an increase in relative expression on all glucose-containing polymers, suggesting that transcriptional activators also play a role in T. maritima response to glucans. The highly expressed ROK family protein TM0393 showed its highest expression on laminarin and starch. Although expressed at a low level, a DeoR family regulator TM1069 showed its highest relative expression on starch, like its B. subtilis and E. coli homologs, which play a major role in the regulation of sugar metabolism genes (34).
All endoglycoside hydrolases were repressed during growth on glucose.
Although most endoglycoside hydrolases and certain exoglycosidases exhibited biochemically specific regulation in response to backbone and
sugar linkage type, several other exoglycosidases displayed unexpected
patterns. For instance, growth on CMC induced expression of genes
encoding a fucosidase (TM0306), a -xylosidase (TM0308), three
-galactosidases (TM0310, TM1192, and TM1195), a
-galactosidase, a
-mannosidase (TM1851), and an
-arabinofuranosidase (TM0281). Some
of these genes (TM0306, TM0310, and TM1851) occur in the vicinity of
genes responsible for the uptake and utilization of CMC (TM0305 and
TM1848) and might be expected to be co-regulated. In other cases,
up-regulation of these genes presumably reflects the complex nature of
polysaccharide structures present in the natural environs of
T. maritima. In any case, it is apparent that annotations of some of these exo-acting glycosidase genes in the T. maritima genome need to be reevaluated.
Overall expression levels of genes encoding the glycosyl transferases
TM0392, TM0744, and TM0895 remained high on glucose as well as on all
- and
-glucan polysaccharides. Whereas TM0392 and TM0744 encode
hypothetical proteins placed in family 4, TM0895 encodes a glycogen
synthase. Growth on glucose also resulted in high overall expression
levels of the gene encoding a lipopolysaccharide biosynthesis
protein, TM0631. Relative expression levels of genes encoding other
glycosyl transferases TM0757 (hypothetical protein), TM1405
(lipopolysaccharide biosynthesis-related), and TM0756
(galactosyltransferase) remained high on glucose as compared with the
- and
-glucan polysaccharides, despite their low overall
expression levels on glucose. Growth on laminarin also resulted in high
overall expression levels of lipopolysaccharide synthesis genes, TM0624
and TM0627.
Growth on CMC affected expression patterns of TM0300-TM0304 and
TM1219-TM1223, two sets of genes annotated as oligopeptide ABC
transporters. The former set of genes lies directly upstream of the
gene encoding the extracellular endoglucanase TM0305 and displayed low
overall expression levels compared with the latter, which
is present in a gene string encoding the extracellular -mannanase, TM1227. The latter set was also overexpressed during growth on barley
glucan. Whereas it could be hypothesized that TM0300-TM0304 are
involved in the transport of
-1,4-linked glucose oligomers and
TM1219-TM1223 are involved in the transport of mixed linkage (
-1,4/
-1,3) glucose oligomers, the latter set remained unaffected during growth on the
-1,3-linked glucose polymer, laminarin.
The motif discovery software AlignACE (35) was used to examine the upstream sequences of genes co-regulated on CMC and barley glucan, revealing a strongly conserved palindromic element upstream of TM1848, TM1223, TM0299, TM0308, and TM1524 with consensus NTGaAAACATTTTCNN (see Table III). This motif shows strong similarity to the LacI family protein CcpA consensus CRE site within the Bacillus/Clostridium group of bacteria (36). Thus, the LacI family regulator TM1218, located downstream of the TM1219-TM1223 gene string, might be involved in local regulation of these ABC transporters and other members of the CMC degradation pathway proposed below. However, matches similar to this consensus are found upstream of genes and gene strings expressed on other sugars, raising the intriguing possibility that a global regulatory mechanism similar to CcpA-mediated carbon catabolite repression (CCR) in B. subtilis might be operating in T. maritima (37). Further work will assess this possibility.
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Based on specific gene expression patterns observed during growth on
CMC, the biochemical characteristics of TM1524 (Cel12A) (38), TM1525
(38), TM1848 (39), and TM0305 (40), the cellular localization of these
proteins (5), and the motifs described, a mechanism for the uptake and
utilization of CMC by T. maritima can be proposed (see Fig.
6A).
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Mannose and Mannan Polysaccharides--
Regulation patterns
relating the monosaccharide mannose to mannan polysaccharides differed
considerably from those observed for glucose and glucan
polysaccharides. A strong correlation in gene regulation was observed
between cells grown on mannose and the complex polysaccharide
galactomannan as well as the mixed polysaccharide glucomannan (cluster
4.4). Thus, unlike glucose, growth on mannose as well as mannan
polysaccharides induced expression of the genes encoding the
endoglycoside hydrolases, TM1227 (man5), TM1751
(cel5A), and TM1752 (cel5B). Whereas
extracellular Man5 strictly degrades -1,4 linkages between mannose
residues, the latter two have been recently characterized as
intracellular glucomannanases (5). The mixed polymer glucomannan
induced expression of all of these genes in addition to genes encoding
the endoglucanases TM0305 (cel74) and TM1524
(cel12A) and the gene encoding the cellobiosyl phosphorylase
TM1848. Growth on galactomannan also induced the putative
-amylase,
TM1650 (amy13B) and an arabinogalactan endogalactosidase, TM1201. The presence of galactose side chains in this substrate induced
the expression of genes encoding the exo-acting debranching enzymes,
TM1192 (gal36), TM1193 (gal42A), and TM1195
(gal42B). Whereas Gal36 hydrolyzes
linked galactose
residues attached to the mannan backbone, Gal42A-B cleave
-linked
galactose end residues. These genes remained unaffected on the
monosaccharide mannose as well as the mixed backbone polymer
glucomannan, which lacks galactose side chain residues. The expression
pattern of the LacI family protein TM1200 is similar to those of
TM1191, TM1192, and TM1195, consistent with the location of TM1200
upstream of the other gene string (4). Thus, TM1200 may act as a local regulator of galactose genes in T. maritima.
The degradation of -linked mannose oligosaccharides appears to be
affected by the intracellular exo-acting glycosidase Man2 (TM1624), the
gene that was induced on both mannan polysaccharides and on mannose.
Overall expression levels of genes encoding a hypothetical glycosyl
transferase (TM0392) and glycogen synthase (TM0895) were comparable
with those observed for glucose and glucan polysaccharides. In
contrast, the gene encoding the hypothetical glycosyl transferase
TM0744 remained repressed during growth on mannose. Very high overall
expression levels of pectate lyase (TM0433) were observed on
glucomannan. Growth on mannose as well as on mannan polysaccharides
resulted in high overall expression levels of the genes encoding the
ABC transporters TM1746-TM1750, putatively annotated as oligopeptide
transporters and located upstream of genes encoding the
glucomannanases, TM1751 and TM1752. Overall expression levels of genes
annotated as sugar ABC transporters (TM1232-TM1234) located downstream
of man5 (TM1227) remained low on all three substrates.
Growth on glucomannan induced the transporter genes TM1220-TM1223,
also co-regulated during growth on CMC and barley glucan. A proposed
pathway for galactomannan utilization is shown in Fig. 6B,
based on the specific gene expression patterns described above, as well
as the biochemical properties and cellular localization of TM1227 (41),
TM1624 (42), TM1192 (42), TM1751 (5), and TM1752 (5).
Among the transcriptional regulators that showed mannose-specific up-regulation were two ROK family proteins, TM0110 and TM0411 (cluster 4.7). Mlc, a ROK family member, is known to negatively regulate three phosphotransferase system (PTS) operons, including the mannose PTS manXYZ (43). However, PTS transporters have not been observed in T. maritima. The LacI family regulator TM1856 was also observed to have its highest expression on mannose, although its overall expression levels and those of TM0411 were low when compared with TM0110. TM0510, a putative manganese-dependent transcriptional regulator, and the response regulator TM0842 had high overall expression consistently across all conditions but showed their highest relative expression levels on mannose. Another response regulator, TM0126, displayed lower overall expression levels but also showed up-regulation on mannose. Whereas the transcriptional regulators described above were largely mannose-specific, the ROK family protein TM1224 and the ribose phosphate isomerase family protein TM1228 (44) were expressed on mannose and mannan polysaccharides. Both TM1224 and TM1228 lie in the same gene string as the endoglycoside hydrolase gene, TM1227 (man5).
Xylose and -Xylan--
Similar to regulation patterns observed
for mannose and mannan polysaccharides, growth on xylose and
-xylan
revealed several sets of co-regulated genes (cluster 4.5). Genes
encoding the extracellular endo-acting xylanases TM0061
(xyl10A) and TM0070 (xyl10B) and the
intracellular exo-acting xylosidase TM0076 (xyl13) displayed comparable expression levels on both substrates and were also up-regulated on the monosaccharide mannose. Very high overall expression levels of the gene encoding the
-glucuronidase (45) TM0055 were observed exclusively during growth on
-xylan, whereas overall expression levels of the gene encoding the acetyl xylan esterase TM0077 were similar across several polysaccharides (CMC,
-1,3/4 glucan, starch) and monosaccharides (mannose and xylose). Relative expression levels of the family 4
-glucuronidase gene TM0434 (32) were high on both xylose and
-xylan, whereas its overall
expression levels were lower than those of other related genes. As
mentioned above, TM0434 was previously classified as a putative
-glucosidase and showed high relative expression levels on starch.
The other acetyl xylan esterase, TM0435, had high relative expression
levels on xylose,
-xylan, and glucomannan, although its overall
expression levels were low on these substrates. Other genes encoding
exo-acting glycosidases that were expressed on both xylose and
-xylan included a putative
-glucosidase (TM0752) and a
-mannosidase (TM1231). Whereas TM0300-TM0303 were highly expressed
on xylose, TM0430-TM0432 were expressed on
-xylan. The proposed
pathway for the utilization of
-xylan is shown in Fig.
6C.
Genes up-regulated on xylan and konjac glucomannan are located in at
least two main genomic regions (cluster 4.6). The region located
between TM0055 and TM0077 revealed some striking compositional similarities to the glucuronic acid utilization cluster of
Bacillus stearothermophilus T-6 (46), consistent with the
identification of side chain substitutions in the form of acetyl,
arabinosyl, ferulic acid and 4-O-methyl--glucuronic acid
in the
-xylan backbone (46). Common features include the presence of
an IclR/KdgR family regulatory protein upstream of the glucuronic acid
metabolism genes kdgAK and uxaBA, and
divergently transcribed from uxaC, and the co-localization
of extracellular and intracellular xylanases within the cluster (4,
46). The second genomic region between TM0430 and TM0439 contains gene
strings sharing sequence homology with genes involved in the related
process of pectin degradation.1 The ABC transporter
subunits TM0430-TM0432 are located upstream of TM0433 (pectate lyase),
whereas TM0434-TM0443 lie downstream within a gene string whose
members are all separated by 20 bases or less. TM0439 is
annotated as a GntR family regulator and is 35% identical and 51%
similar to the gluconate repressor GntR of B. subtilis
(47).
Other putative regulators showed expression patterns suggesting involvement in the regulation of xylose and xylan-related genes. TM0949 displays 30% identity with E. coli XylR, the positive regulator of the xylFGH ABC transporter operon, and showed a particularly strong induction on xylose. ABC transporters located downstream of TM0949 show similarity to xylFGH, and it remains to be seen whether these genes, annotated as ribose ABC transporters, are induced during growth on xylose as observed in E. coli (18, 48). The CRP-like transcriptional regulator TM1171 was up-regulated primarily on xylose but to a lesser extent on CMC. Experimental evidence in Thermotoga neapolitana suggests that CCR in that species is not dependent on cAMP (49). If this is also the case in T. maritima, the function of the CRP protein may differ from that of E. coli CRP, whose role in global catabolite repression has been well documented (50). TM0808, the single ROK family protein that showed increased expression on xylan and xylose, is located upstream of a putative hydrolase (TM0809), which showed expression preferences for xylan and starch.
Summary--
The genome sequence of T. maritima
encodes for the highest number of known glycoside hydrolase genes among
hyperthermophilic bacterial and archaeal genome sequences reported to
date (Fig. 1) (51). The data set resulting from this study provides a
rich resource for characterizing putative regulatory mechanisms in T. maritima that might have been proposed from genome
sequence data (4). Previous studies on T. neapolitana have
reported the classical lactose-glucose diauxie, operating independently of cAMP (49). In the case of -galactoside transport and metabolism in T. neapolitana, transport system-based regulation through
inducer exclusion or expulsion has been ruled out (52). The results presented here indicate the presence of carbon catabolite repression during growth on glucose. However, growth on this substrate was observed to be slower than on any of the glucan polysaccharides. Growth
on the other monosaccharides, xylose and mannose, did not repress the
genes responsible for the uptake and utilization of the corresponding
polysaccharides, pointing to similar modes of regulation between these
mono- and polysaccharide substrates. Studies on glucose uptake by
T. neapolitana propose that transport of this monosaccharide
is energized by ion gradients generated by ATP, derived from substrate
level phosphorylation (2). It remains to be seen whether ABC
transporters identified in this study that were expressed during growth
on mannose and mannan polysaccharides follow similar mechanisms. The
observation that the expression patterns of transcriptional regulators
group with genomic neighbors although cDNA spots were randomized on
the array provides verification that systematic biases that can obscure co-regulation have been removed from the microarray data during analysis. Results from this study suggest sugar-specific regulation patterns for members of several large protein families in T. maritima, including the ROK (20) and LacI (21) families that were
not apparent from sequence data alone. Protein sequence conservation, genomic organization, and microarray expression patterns can be combined to predict local regulatory sequences for many T. maritima proteins.3
Sequence similarity of several T. maritima LacI family proteins to the CcpA protein involved in CCR in B. subtilis coupled with the CRE-like (catabolite-responsive element) sequence motif found upstream of several sugar specific operons suggests that a type of global carbon catabolite repression may be occurring in T. maritima.3 This mechanism is likely to differ from that operating in B. subtilis, since the T. maritima genome does not contain a strong homolog to either HPr or Crh, and no PTS systems have been identified in this organism (4, 53). In Streptomyces coelicolor, which also lacks HPr, the ROK family protein glucokinase has been implicated in yet another global regulatory mechanism of CCR (37). The expression of the T. maritima CRP protein homolog observed in this microarray study raises even more questions about available mechanisms for CCR in T. maritima.
Further studies using full genome microarrays and biochemical studies
to unravel the pathways for intracellular metabolism in this organism
with presumably primitive traits are currently under way.
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ACKNOWLEDGEMENTS |
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We acknowledge Dr. Bryon Sosinski and Dr. Len van Zyl for helpful discussions.
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FOOTNOTES |
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* This work was supported in part by grants from the National Science Foundation (NSF) Biotechnology Program, an NSF IGERT Traineeship (to S. B. C.), and the Department of Energy Energy Biosciences Program.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
The on-line version of this article (available at
http://www.jbc.org) contains two additional tables.
¶ To whom correspondence should be addressed: Dept. of Chemical Engineering, North Carolina State University, 113 Riddick Lab, Box 7905, Raleigh, NC 27695-7905. Tel.: 919-515-6396; Fax: 919-515-3465; E-mail: rmkelly@eos.ncsu.edu.
Published, JBC Papers in Press, December 9, 2002, DOI 10.1074/jbc.M211748200
1 L. D. Kluskens, personal communication.
3 S. B. Conners and R. M. Kelly, unpublished data.
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ABBREVIATIONS |
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The abbreviations used are: ANOVA, analysis of variance; CMC, carboxymethylcellulose; PTS, phosphotransferase system; CCR, carbon catabolite repression.
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