Altered morphology produced by ftsZ expression in Corynebacterium glutamicum ATCC 13869

Angelina Ramos1,{dagger}, Michal Letek1, Ana Belén Campelo1, José Vaquera2, Luis M. Mateos1 and José A. Gil1

1 Departamento de Ecología, Genética y Microbiología, Área de Microbiología, Facultad de Biología, Universidad de León, 24071 León, Spain
2 Departamento de Biología Celular, Facultad de Biología, Universidad de León, 24071 León, Spain

Correspondence
José A. Gil
degjgs{at}unileon.es


   ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Corynebacterium glutamicum is a Gram-positive bacterium that lacks the cell division FtsA protein and actin-like MreB proteins responsible for determining cylindrical cell shape. When the cell division ftsZ gene from C. glutamicum (ftsZCg) was cloned in different multicopy plasmids, the resulting constructions could not be introduced into C. glutamicum; it was assumed that elevated levels of FtsZCg result in lethality. The presence of a truncated ftsZCg and a complete ftsZCg under the control of Plac led to a fourfold reduction in the intracellular levels of FtsZ, generating aberrant cells displaying buds, branches and knots, but no filaments. A 20-fold reduction of the FtsZ level by transformation with a plasmid carrying the Escherichia coli lacI gene dramatically reduced the growth rate of C. glutamicum, and the cells were larger and club-shaped. Immunofluorescence microscopy of FtsZCg or visualization of FtsZCg–GFP in C. glutamicum revealed that most cells showed one fluorescent band, most likely a ring, at the mid-cell, and some cells showed two fluorescent bands (septa of future daughter cells). When FtsZCg–GFP was expressed from Plac, FtsZ rings at mid-cell, or spirals, were also clearly visible in the aberrant cells; however, this morphology was not entirely due to GFP but also to the reduced levels of FtsZ expressed from Plac. Localization of FtsZ at the septum is not negatively regulated by the nucleoid, and therefore the well-known occlusion mechanism seems not to operate in C. glutamicum.


Abbreviations: DAPI, 4',6-diamino-2-phenylindole

{dagger}Present address: Departamento de Biología Funcional, Área de Microbiología, Facultad de Medicina, Universidad de Oviedo, 33006 Oviedo, Spain.


   INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
FtsZ is one of the most conserved cell division proteins and it have been found in all bacteria examined [except Chlamydia trachomatis, Ureaplasma urealyticum (Dziadek et al., 2002), and crenarchaea (Margolin, 2000)], chloroplasts (Osteryoung & Vierling, 1995) and certain mitochondria (Osteryoung, 2001). It has been suggested that the primary role of FtsZ is to cause invagination of the cytoplasmic membrane. This hypothesis is supported by the facts that FtsA (Ma et al., 1996) and ZipA (Hale & de Boer, 1997, 2002) bind directly to FtsZ polymers at the future division site, followed by the sequential addition of FtsK, FtsQ, FtsL, FtsW, FtsI and FtsN.

Heterologous expression of the ftsZ gene from different micro-organisms in Escherichia coli leads to filamentation, probably because heterologous FtsZ interferes with the resident FtsZ (Honrubia et al., 1998; Margolin et al., 1991; Salimnia et al., 2000; Yaoi et al., 2000). This was confirmed by the introduction of FtsZ–GFP from Rhizobium (Sinorhizobium) meliloti into E. coli cells, which resulted in the formation of ring structures, suggesting co-localization with the E. coli FtsZ within non-functional division rings (Ma et al., 1996). However, when the level of ftsZ expression was low, heterologous FtsZ co-localized with endogenous FtsZ, forming a functional FtsZ ring, and the cells divided normally.

The homologous expression of ftsZ genes has mainly been studied in E. coli, where it has been shown that a two- to sevenfold increase in the level of the FtsZ protein causes increasing numbers of additional septa to form near the cell poles, producing minicells, and that an increase in the level of FtsZ beyond this range results in complete inhibition of cell division (Ward & Lutkenhaus, 1985). Plasmid pZAQ, carrying the complete ftsQ, ftsA and ftsZ genes from E. coli, increased FtsZ levels by about sevenfold and increased the frequency of both polar and centrally located septa (Begg et al., 1998; Bi & Lutkenhaus, 1990). The increase in both types of septa was originally attributed to increased FtsZ level, but it was later shown that an increase in both FtsZ and FtsA is required to cause early central division (Begg et al., 1998).

Homologous overexpression of FtsZ has been also studied in Neisseria gonorrhoeae (Salimnia et al., 2000), Halobacterium salinarum (Margolin et al., 1996) and Mycobacterium tuberculosis (Dziadek et al., 2002), among others. In N. gonorrhoeae, the expression of FtsZ resulted in abnormal cell division in some cells, characterized by the presence of multiple and atypically arranged cell division sites. H. salinarium transformed with a plasmid carrying its own ftsZ gene yielded few large but many small transformant colonies. Cells from small colonies contained the original plasmid, whereas the plasmid isolated from large colonies had undergone an internal deletion, leading to an inactivated ftsZ gene; this indicated that ftsZ expression was deleterious. Furthermore, cells from large colonies were morphologically of the wild-type (containing only the chromosomal ftsZ gene), whereas cells from small colonies were pleomorphic (Margolin et al., 1996). In M. tuberculosis, unregulated expression of ftsZ from constitutive promoters resulted in lethality, and when a small number of transformants was obtained, plasmids isolated from those transformants displayed deletions in the ftsZ coding region (Dziadek et al., 2002).

When we attempted to study the homologous expression of ftsZ in Corynebacterium glutamicum using different high-copy-number vectors and different transformation methods we consistently failed to obtain transformants. Because our goal was to manipulate cell division genes in this industrially important micro-organism, we constructed a strain carrying a truncated ftsZ gene and a complete ftsZ under the control of a known promoter. Here we describe the morphological changes observed.


   METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Bacterial strains and plasmids.
All strains and plasmids used are listed in Table 1.


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Table 1. Bacterial strains and plasmids

 
DNA isolation and manipulation.
Plasmid DNA was isolated from E. coli and from corynebacteria using the alkaline lysis method described for Streptomyces (Kieser, 1984) but treating corynebacterial cells with lysozyme for 2–3 h at 30 °C.

Total DNA from corynebacteria was isolated using the Kirby method described for Streptomyces (Kieser et al., 2000), but cells were treated with lysozyme for 4 h at 30 °C.

Samples of total DNA from different C. glutamicum transconjugants were digested with EcoRI and hybridized separately with a 789 bp NdeI–BamHI fragment from the C. glutamicum ftsZ gene (ftsZCg) isolated from plasmid pPHEZ4 and with a 1·4 kb HincII fragment from plasmid pULMJ8 carrying kan. Both fragments were labelled with digoxigenin according to the manufacturer's (Boehringer Mannheim) instructions.

RNA from different C. glutamicum strains was isolated at different culture times in TSB media using the RNeasy kit (Qiagen). For Northern experiments, 20 µg RNA was loaded into a 1·5 % formaldehyde-agarose gel and transferred to nylon membranes. Filters were hybridized with an internal fragment of ftsZCg (513 bp HincII fragment) from C. glutamicum radiolabelled by nick-translation.

Plasmid constructions.
To express ftsZCg in C. glutamicum, a 2·1 kb BglII fragment from the C. glutamicum chromosome (obtained from plasmid pPHEZQ1; see Table 1) containing the whole ftsZ gene and upstream (358 nt) and downstream (433 nt) sequences was cloned into the unique BglII site of plasmid pUL880M (Adham et al., 2001b) or into the high-copy-number conjugative bifunctional plasmid pECM2 (Jager et al., 1992), creating plasmids pBZ81 and pECZ1 respectively (Table 1). These plasmids were constructed in E. coli and transferred to C. glutamicum by electroporation or conjugation respectively. The copy number of plasmid pECM2 and derivatives was estimated as 30–40 copies per cell by densitometry as described previously (Santamaria et al., 1984).

To introduce a second copy of ftsZCg into the chromosome of C. glutamicum, a 3 kb EcoRV–Ecl136II fragment from plasmid pPHEZQ1 (carrying ftsQ, ftsZ and the 5'-end of yfiH) (Honrubia et al., 2001) was cloned into pK18mob (Schafer et al., 1994) digested with NdeI (Klenow filled), creating plasmid pIZ1 (Table 1).

Plasmid pKZLac, used to disrupt the chromosomal copy of ftsZCg and to introduce a ftsZCg gene under the control of Plac, was constructed as follows: a 789 bp NdeI (Klenow-filled)–BamHI fragment from plasmid pPHEZ4 (Table 1) encoding the first 263 amino acids from FtsZ was cloned into the SmaI–BamHI sites of pK18mob (Table 1).

Plasmid pK18-3{Delta}Z was constructed by cloning a 1146 bp BamHI (Klenow-filled) fragment from plasmid pPHEZ1 (Table 1) into NdeI-digested and Klenow-filled plasmid pK18-3 to integrate, by single recombination, the deleted ftsZ into one of the three ORFs located downstream from ftsZ.

To make a ftsZ–gfp translational fusion, the 3' end of ftsZ was amplified by PCR using primers Zgfp-1 (5'-GCAACCATGGACGGCGCAACTGGCGTCCTG-3') and Zgfp-3 (5'-CCCTTAAGCATATGGAGGAAGCTGGGTACATCCAGGTCG-3'). These primers were designed to replace the two final codons of ftsZ [CAG (Gln) and TAA (stop)] by an NdeI site (CAT ATG) (His and Met). All PCR reactions were performed as described previously (Adham et al., 2003) and the amplified fragment was digested with BamHI (present in the amplified fragment) + NdeI (CATATG) and cloned together with gfp (as a NdeI–XbaI fragment) into plasmid pET28a, creating pETGFP (Table 1). Therefore the last amino acid of FtsZCg will be His instead of Gln, and fused to GFP. The gfp gene used was egfp2 (Enhanced green fluorescent protein) from Clontech, including the V163A and S175G mutations introduced by Siemering et al. (1996). The in-frame fused 'ftsZ–gfp gene was isolated from pETGFP as a BamHI–XbaI fragment, sequenced (see later), and cloned into plasmid pK18mob (digested with BamHI and XbaI), to give plasmid pKZGFP (Table 1), which was introduced by conjugation into C. glutamicum and integrated into its chromosome by single recombination.

Plasmid pK18-3ZG was constructed by cloning a 1·2 kb EcoRV–BamHI fragment from pPHEZQ1 (carrying ftsQ, ftsZ') and the 'ftsZ–gfp gene isolated from pETGFP as a BamHI–XbaI (Klenow filled) fragment into NdeI-digested and Klenow-filled plasmid pK18-3 to integrate, by single recombination, ftsZ–gfp into one of the three ORFs located downstream from ftsZ.

DNA sequence determination.
When necessary, both strands of the plasmids constructed were sequenced either manually or with an ALF automated sequencing apparatus (Pharmacia), using specific primers.

Preparation of cell-free extracts, PAGE and Western blotting.
Cell-free extracts of C. glutamicum were prepared by sonication as described previously (Ramos et al., 2003b). SDS-PAGE was carried out essentially as described by Laemmli (1970). After electrophoresis, proteins were stained with Coomassie blue or electroblotted to PVDF membranes (Millipore) and immunostained with anti-FtsZ polyclonal antibodies raised against purified His-tagged FtsZ from C. glutamicum (Honrubia et al., 2005). The dilution of the anti-FtsZ antibodies used was 1 : 10 000.

Microscopic techniques.
Scanning electron microscopy and fluorescence microscopy of C. glutamicum cells were performed according to protocols described previously (Ramos et al., 2003b). Staining with DAPI (4',6-diamino-2-phenylindole) and immunofluorescence microscopy was performed as described by Daniel et al. (2000), except that C. glutamicum cells were permeabilized overnight at 30 °C with lysozyme at a final concentration of 10 mg ml–1. Permeabilized cells were incubated overnight at 4 °C with 1 : 10 000 dilution of the above-mentioned anti-FtsZCg antiserum. Anti-rabbit fluorescein isothiocyanate conjugate (Sigma) at 1 : 10 000 dilution was used as secondary antiserum.


   RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Overexpression of ftsZCg is lethal in C. glutamicum, as in E. coli
It has been described that expression of ftsZCg in E. coli is lethal and leads to filamentation (Honrubia et al., 1998) whereas ftsZCg overexpression in E. coli leads to the creation of deleted and innocuous variants of FtsZCg (Honrubia et al., 2005). In order to study the effect of multiple copies of ftsZCg in C. glutamicum, the bifunctional plasmid pBZ81 [a pUL880M derivative containing the whole ftsZ gene and upstream (358 nt) and downstream (433 nt) sequences] was introduced into C. glutamicum by electroporation or by using protoplasts in the presence of PEG; after several transformation and electroporation experiments, no transformants were obtained in any case. Control experiments using pUL880M were always successful (105 transformants per µg DNA). Occasionally, a small number (<10 transformants per µg DNA) of hygromycin-resistant transformants was obtained, but these transformants bore plasmids that had undergone rearrangements or deletions. Similar results were obtained by Dziadek et al. (2002) when expressing M. tuberculosis FtsZ in M. tuberculosis under the control of different constitutive promoters.

Because, at least in our hands when using C. glutamicum R31, conjugation was a more efficient method than transformation or electroporation for introducing plasmid DNA into C. glutamicum, plasmid pECZ1 [a pECM2 derivative containing the whole ftsZ gene and upstream (358 nt) and downstream (433 nt) sequences] was introduced into C. glutamicum cells by conjugation. No C. glutamicum transconjugants were obtained, indicating the lethality of the ftsZ gene in C. glutamicum when cloned in high-copy-number vectors. As above, control experiments using pECM2 were always successful (103–104 transconjugants per 107 donor E. coli cells).

Introduction of a second copy of ftsZ into the chromosome of C. glutamicum
pUL880M and pECM2 are multicopy plasmids (approx. 30 copies per cell) derived from the endogenous plasmids pBL1 (Santamaria et al., 1984) and pCG1 (Jager et al., 1992) respectively. Therefore, the high level of FtsZ produced might be responsible for the lethality of FtsZ as described previously (Margolin et al., 1996). Because of the lack of low-copy-number vectors for corynebacteria, we introduced a second copy of ftsZ into the genome of C. glutamicum by homologous recombination. The conjugative suicide plasmid pIZ1 (Table 1) was introduced into C. glutamicum by conjugation. Kanamycin-resistant transconjugants were morphologically wild-type, and after Southern blot analysis (not shown) it was confirmed that a second copy of ftsZ was present in their genome. One of these transconjugants was named C. glutamicum AR1 (Table 1, Fig. 1). We therefore conclude that two copies of ftsZ under the control of their own promoters do not negatively affect the morphology or the viability of C. glutamicum.



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Fig. 1. Schematic representation of the chromosomal DNA regions around the ftsZ gene in the different C. glutamicum strains constructed in this work. Arrows represents ORFs. 'gen and gen' represent a gene lacking the 5'-end or the 3'-end respectively. egfp2 is the gene encoding the GFP used in this work. Plac is the promoter of the E. coli lac operon. Wavy lines represent a plasmid inserted in the chromosomal DNA carrying the kanamycin (kan) or the apramycin (apr) resistance gene.

 
Construction of C. glutamicum strains carrying a unique complete copy of ftsZ under the control of Plac
To confirm the lethality of high levels of ftsZCg expression in C. glutamicum, we disrupted the chromosomal copy of ftsZ and replaced it by an ftsZCg gene under the control of the lac promoter (Plac) from E. coli. This promoter was chosen because it has been described as an efficient promoter in C. glutamicum (Tsuchiya & Morinaga, 1988).

Plasmid pKZLac was introduced by conjugation into C. glutamicum and transconjugants were selected by resistance to kanamycin. Southern analysis of the transconjugants showed the expected DNA pattern, which confirmed the integration of pKZLac by single recombination. The resulting strain has a partial ftsZ gene (capable of expressing a C-terminally truncated product of 263 amino acids) in the original chromosomal position and a copy of ftsZ (438 amino acids) under the control of Plac. One of these transconjugants was named C. glutamicum AR2 (Fig. 1). We assumed that the partial ftsZ would encode a non-functional FtsZ, whereas Plac would direct the expression of the complete ftsZ gene. The transconjugants showed a slower growth rate than that of C. glutamicum R31, and an aberrant morphology was observed by phase-contrast microscopy (Fig. 2a) and scanning electron microscopy (Fig. 2b).



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Fig. 2. (a, b) Phase-contrast microscopy (a) and scanning electron microscopy (b) of C. glutamicum strains R31, AR2 and AR20. (c) Western blot analysis of total protein extracts obtained from these three strains probed with polyclonal anti-FtsZCg antibody. The amount of protein loaded was 1 µg and it was obtained from cultures at an OD600 of 1.

 
To rule out the possibility that this aberrant morphology might be due to the expression of the partial ftsZ gene, still present in the chromosome of C. glutamicum AR2 (see Fig. 1), plasmid pK18-3{Delta}Z (Table 1) was introduced by conjugation into C. glutamicum to insert the partial ftsZ gene in a non-essential chromosomal region using a strategy designed to introduce any gene into the C. glutamicum chromosome (Adham et al., 2001a). The new strain, C. glutamicum AR12, contained the partial ftsZ and the original ftsZ under the control of their endogenous promoters (Fig. 1). This strain was morphologically wild-type and we therefore concluded that the aberrant morphology of C. glutamicum AR2 must be due to the expression of ftsZ under the control of Plac.

Because Plac has been described to be an inducible promoter in C. glutamicum (Tsuchiya & Morinaga, 1988), and because there is no lacI, nor lac operon, in the genome of C. glutamicum (A. Ramos, unpublished), we assumed that ftsZ was being overexpressed and that the aberrant morphology observed was due to high levels of FtsZCg. To confirm this hypothesis, RNA was isolated from exponentially growing cells of C. glutamicum R31 and AR2 and hybridized with an internal fragment of ftsZCg. The amount of specific mRNA for ftsZ in C. glutamicum R31 (measured by densitometry) was roughly 1·5–2 times higher than the level of mRNA for ftsZ in C. glutamicum AR2 (data not shown). Western blot experiments with polyclonal anti-FtsZCg antibodies gave similar results (four times more FtsZ protein in C. glutamicum R31 than in C. glutamicum AR2 measured by densitometry) (Fig. 2c). In sum, filamentous cells containing buds, knots and branch-like outgrowths are obtained at FtsZ concentrations fourfold below physiological levels.

To see the phenotypic effect of much more reduced ftsZCg expression from Plac, the lacIq gene from E. coli present in plasmid pECKX99E (30 copies per cell) (Table 1) was introduced into C. glutamicum AR2A (Table 1). The resulting strain (C. glutamicum AR20) showed a more reduced level of FtsZ (20 times less FtsZ than C. glutamicum R31) (Fig. 2c), a marked globular morphology (Fig. 2a, b) and a much slower growth rate than C. glutamicum AR2 (Fig. 3, Table 2).



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Fig. 3. Growth kinetics of C. glutamicum strains R31 ({blacktriangleup}), AR2 ({blacksquare}) and AR20 ({bullet}) in TSA medium (Difco) without kanamycin (for R31) or with 25 µg kanamycin ml–1 (for AR2 and AR20).

 

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Table 2. Viable counts of three C. glutamicum strains at various OD600 values

 
Visualization of FtsZCg in C. glutamicum
Taking into account previous results concerning the lethality of FtsZCg overexpression in C. glutamicum, in order to visualize FtsZ in living cells we constructed a strain [C. glutamicum AR5 (Fig. 1)] bearing ftsZ–gfp as a single copy in the chromosome, using the conjugative suicide plasmid pKZ-GFP (Table 1). The localization of FtsZCg–GFP was characterized in live C. glutamicum AR5 (Fig. 4) cells from exponential-phase liquid cultures (OD600 1). Phase-contrast microscopy revealed that cells were twice as large as the wild-type, as has been described for E. coli (Sun & Margolin, 1998). Fluorescence microscopy revealed that most of the cells showed a fluorescent band, probably a ring, at the mid-cell; these bands were not detected by phase-contrast microscopy, indicating that they were not insoluble inclusions but true cytoskeletal structures. These cells were probably in division, suggesting that FtsZ–GFP polymerizes and permits septation. It was also possible to observe larger cells containing two FtsZ rings in the mid-cell of the future daughter cells. It may be concluded that FtsZ–GFP is mostly functional because it can completely replace wild-type FtsZCg with no significant morphological problems. The apparent off-centre localization of the FtsZ ring is not unexpected because C. glutamicum does not divide symmetrically (M. Letek, unpublished).



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Fig. 4. FtsZ distribution in C. glutamicum strains AR5 (ftsZ–gfp as a single copy in the chromosome), AR50 (Plac–ftsZ–gfp as a single copy in the chromosome), ML13 (merodiploid) and AR2. Note that the pictures of C. glutamicum AR5, AR50 and ML13 were taken by fluorescence microscopy (FtsZ–GFP) whereas those of C. glutamicum AR2 were taken by immunofluorescence microscopy. Phase-contrast microscopy of C. glutamicum R31 was included as control. Scale bars, 2 µm.

 
Visualization of FtsZCg in C. glutamicum carrying ftsZ under the control of Plac
Plasmid pOJZ-GFP (a suicide conjugative plasmid containing an apramycin resistance gene and the in-frame fused 'ftsZ–gfp gene; Table 1) was introduced by conjugation from E. coli into C. glutamicum AR2 and transconjugants were selected by resistance to kanamycin and apramycin. Two types of transconjugants were anticipated: those that would arise by single recombination between pKZlac previously integrated into the chromosome of C. glutamicum AR2 and pOJZ-GFP, and those integrated at the 3'-end of ftsZCg under the control of Plac. Thirty-four kanamycin- and apramycin-resistant transconjugants were obtained, and two of these showed fluorescence when observed under the fluorescence microscope. Total DNA was isolated from these transconjugants, and both of them exhibited the expected Southern hybridization pattern (not shown). One of these transconjugants was named C. glutamicum AR50 and its genetic structure around ftsZ was determined (Fig. 1).

Fluorescence microscopy revealed that most of the C. glutamicum AR50 cells showed one or two fluorescent bands, probably rings, at the mid-cell, and these bands were true cytoskeletal structures (Fig. 4). Spirals or multiple FtsZ septa were also present in some aberrant cells.

Because in C. glutamicum AR5 and C. glutamicum AR50 the only functional copy of FtsZ is fused to GFP, we constructed a merodiploid strain (C. glutamicum ML13) carrying a functional ftsZ allele in addition to the ftsZ–gfp fusion. In such contexts, the fusions often interfere less with division or growth and may give more reliable localization data. Fluorescence microscopy of C. glutamicum ML13 revealed that FtsZ–GFP localizes mainly in the septum, and because the growth rate of this strain is normal, we can conclude that FtsZCg–GFP copolymerizes with FtsZCg and permits septation.

The nucleoid does not inhibit the localization of FtsZ at mid-cell in C. glutamicum
To confirm the localization data of FtsZ obtained using the merodiploid strain C. glutamicum ML13, immunofluorescence microscopy was used to determine the localization of FtsZ in exponentially growing cells from C. glutamicum strains R31 and AR2, the parental strains of C. glutamicum AR5 and C. glutamicum AR50 respectively (see Fig. 1). Immunofluorescence microscopy of C. glutamicum R31 revealed that most of the cells showed a fluorescence band at the mid-cell overlapping with the nucleoid (Fig. 5, vertical arrows). These results might indicate that the assembly of the FtsZ ring at the cell division site in C. glutamicum is not negatively regulated by the nucleoid as in E. coli (Sun & Margolin, 2004) or Bacillus subtilis (Wu & Errington, 2004). Two FtsZ rings can be observed after nucleoid segregation (Fig. 5, horizontal arrows) and a reduced polar accumulation of FtsZ in some small, probably just replicated, cells which is interpreted as a remnant of the previous FtsZ ring.



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Fig. 5. FtsZ and nucleoid distribution in C. glutamicum R31. (a) FtsZ immunofluorescence staining. (b) DAPI staining. (c) Overlay of (a) and (b). Vertical arrows indicate single FtsZ rings (in panel a) and DAPI-stained nucleoid (in b). The horizontal arrow marks a dividing cell with two possible FtsZ rings and two separated nucleoids. Note the presence of apical FtsZ foci at one end of some just divided cells (arrowheads in a).

 
Immunofluorescence microscopy of C. glutamicum AR2 (Fig. 4) confirmed the presence of FtsZ rings or spirals when the expression of FtsZ takes place from Plac.


   DISCUSSION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
It is becoming clear that bacteria have a cytoskeleton composed of structural homologues of tubulin and actin. This cytoskeleton is responsible for cell growth, division and shape. FtsZ is the structural homologue of tubulin, and FtsA/MreB/MreB-like protein (Mbl) are the structural homologues of actin. FtsZ and FtsA are required for cell division, whereas MreB and Mbl are involved in cell shape and localize as a helical filament that guides dispersed cell wall biosynthesis (Daniel & Errington, 2003; Margolin, 2003).

Corynebacteria are Gram-positive micro-organisms that lack both the FtsA and MreB systems and therefore lack the structural homologues of actin. How, then, do corynebacteria control cell shape, division and growth in the absence of a typical actin homologue?

C. glutamicum formed elongated, club-shaped or dumbbell-shaped rods, but not filaments, in the presence of antibiotics that inhibit septation or DNA synthesis (Kijima et al., 1998), and similar cells were obtained when a single copy of ftsZCg was expressed under the control of Plac in C. glutamicum AR2 (see Fig. 2). The presence of lacI (C. glutamicum AR20) led to a reduced growth rate and to the presence of more cells with a club-shaped morphology. This phenotype is very similar to that obtained when divIVACg is overexpressed in C. glutamicum (Ramos et al., 2003b); the main difference between the two phenotypes is the inhibition of cell division when the expression of FtsZ is diminished. C. glutamicum AR2 and C. glutamicum R31 reached the same optical density after 96 h of growth, but the number of viable cells was one or two orders of magnitude higher in R31 than in AR2 for the same OD600 (Table 2).

Our results concerning the expression of ftsZCg under Plac are in partial agreement with the results obtained in Rhizobium (Sinorhizobium) meliloti (another rod-shaped bacterium that lacks MreB homologues) (Latch & Margolin, 1997) and Mycobacterium tuberculosis (Dziadek et al., 2002). It may be concluded that in rod-shaped micro-organisms lacking MreB, branching and swelling are default pathways for increasing mass when cell division is blocked either by FtsZ overproduction (Latch & Margolin, 1997) or by partial depletion of FtsZ (this work).

Daniel & Errington (2003) used fluorescent vancomycin to determine the spatial pattern of peptidoglycan biosynthesis in C. glutamicum, and observed that cell division septa and cell poles were labelled by vancomycin. The labelling of cell division septa confirms the notion that peptidoglycan synthesis takes place at the septum, but the labelling of cell poles implies that peptidoglycan biosynthesis occurs mainly at the poles in the absence of the helical scaffold MreB. This type of apical growth and peptidoglycan biosynthesis at the cell poles has been described previously for Corynebacterium diphtheriae (Umeda & Amako, 1983). Two different proteins have been found localized at the cell poles of C. glutamicum: an inorganic pyrophosphatase (PPase) and DivIVACg (Ramos et al., 2003a, b). Pyrophosphate is a by-product in the biosynthesis of UDP-N-acetylglucosamine by UDP-N-acetylglucosamine pyrophosphorylase (EC 2.7.7.23) and hydrolysis of pyrophosphate by PPase is essential for cell wall biosynthesis (Ramos et al., 2003a). DivIVACg might form oligomeric structures (perhaps through its coiled-coil regions) with a possible structural function at C. glutamicum growing cell poles, and probably accumulates there through interaction with proteins involved in peptidoglycan biosynthesis. Is DivIVACg the scaffold for peptidoglycan biosynthesis in MreB-lacking rod-shaped corynebacteria? DivIVACg seems to be an essential protein, and therefore it was not possible to disrupt divIVACg (Ramos et al., 2003b). Further experiments will be needed to deplete DivIVACg in order to study its possible effect on cell morphology and peptidoglycan biosynthesis by vancomycin staining.

Microscopic studies of C. glutamicum carrying a chromosomal ftsZCg–gfp gene fusion under the control of the ftsZCg endogenous promoters clearly indicated that these cells were probably in division, suggesting that FtsZCg–GFP polymerizes and permits septation. Spirals of FtsZ have been observed in E. coli overexpressing FtsZ–GFP (Ma et al., 1996) as well as in B. subtilis in the switch from medial to asymmetric division prior to sporulation (Ben Yehuda & Losick, 2002). In contrast, spirals were observed in C. glutamicum when ftsZ was expressed at low levels.

It was also possible to see small polar aggregates of FtsZ in C. glutamicum R31 by immunofluorescence microscopy that may represent FtsZ left from the previous division or a polar Z ring involved in the next division cycle, because C. glutamicum seems to grow in a polar fashion (Daniel & Errington, 2003).

In contrast to E. coli or B. subtilis, the localization of the FtsZ ring at the mid-cell in C. glutamicum is not negatively regulated by nucleoid occlusion (see Fig. 5), nor by the MinCD system, since no homologue of MinCD has been detected in the C. glutamicum genome (Ramos et al., 2003b). It is becoming clear that corynebacteria have a mechanism of cell division and peptidoglycan synthesis different from that in other classical rod-shaped bacteria and well-known model organisms; therefore the study of cell division in corynebacteria may reveal a different model of cell division and peptidoglycan biosynthesis.


   ACKNOWLEDGEMENTS
 
This work was funded by grants from the Junta de Castilla y León (ref. LE 24/01), the University of León (ULE 2001-08B) and the Ministerio de Ciencia y Tecnología (BIO2002-03223). A. Ramos, A. B. Campelo and M. Leteck were recipients of fellowships from the Junta de Castilla y León, the Diputación Provincial de León, and the Ministerio de Educación y Ciencia respectively.


   REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Received 15 March 2005; revised 11 May 2005; accepted 12 May 2005.



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