From the Lehrstuhl für Biochemie der Pflanzen,
Fakultät für Biologie, Ruhr-Universität Bochum,
Universitätsstra
e 150, D-44780 Bochum, Germany and the
§ Glynn Laboratory of Bioenergetics, Department of
Biology, University College London, Gower Street, London WC1E 6BT,
United Kingdom
Received for publication, October 18, 2000, and in revised form, January 31, 2001
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ABSTRACT |
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To investigate the function of the PetM subunit
of the cytochrome b6f complex, the
petM gene encoding this subunit was inactivated by
insertional mutagenesis in the cyanobacterium Synechocystis PCC 6803. Complete segregation of the mutant reveals a nonessential function of PetM for the structure and function of the cytochrome b6f complex in this organism.
Photosystem I, photosystem II, and the cytochrome
b6f complex still function normally
in the petM The cytochrome b6f complex is an
essential component of the electron transport chain in oxygenic
photosynthetic organisms. In green algae and higher plants it is
located in the thylakoid membrane of chloroplasts and mediates electron
transport from PS1 II to PS I
(for reviews see Refs. 1 and 2). In cyanobacteria (3), it is located
both in thylakoid and cytoplasmic membranes (4). Because of the absence
of a cytochrome bc1 complex, the cytochrome
b6f complex fulfills additional
functions in these organisms. It is involved in both
photosynthetic and respiratory electron transport, acting as a
plastoquinol-cytochrome c6-plastocyanin oxidoreductase and playing a role in electron transfer from PS II or
NAD(P)H dehydrogenase to PS I or cytochrome oxidase, respectively (5).
In all organisms capable of oxygenic photosynthesis the cytochrome
b6f complex consists of four major
proteins and additional small subunits. The 25-kDa cytochrome
b6 protein contains two b-type hemes and,
together with the 17-kDa protein subunit IV, is homologous to
cytochrome b of the cytochrome bc1
complex (6). Cytochrome b6 and subunit IV are
integral membrane proteins with four and three predicted transmembrane
Whereas the three-dimensional structure of the whole cytochrome
b6f complex has not yet been resolved
at high resolution, the structure of some fragments is known. The
lumenal domain of cytochrome f of Chlamydomonas
reinhardtii (7) and Phormidium laminosum (8) has been
resolved up to 1.9 Å, and the structure of the lumenal domain of the
Rieske protein from spinach has been resolved up to 1.83 Å (9).
Analysis of two-dimensional crystals of the whole complex has led to a
projection map of only 8-Å resolution to date (10). However, the
structure of the cytochrome bc1 complex from
mitochondria has been determined to about 3-Å resolution (11-13), and
this structure is a useful guide for predicting the general structure
of homologous subunits of the cytochrome
b6f complex (14).
Because the core structures of bc1 and
b6f complexes consist of similar
proteins and are functionally nearly identical, their three-dimensional
structures are likely to be very similar. However, the additional
subunits, PetG (15), PetL (16), and PetM (17) in
b6f complexes and up to eight
additional subunits in eukaryotic bc1 complexes,
have no obvious relation with each other and are likely therefore to be
structurally quite different. The functions of the additional small
b6f subunits is not yet known.
Interestingly, a gene encoding the PetL protein cannot be found in the
completely sequenced genome of the cyanobacterium
Synechocystis PCC 6803 (18). Recently the open reading frame
ycf6 of chloroplasts, for which a cyanobacterial homolog
exists, was shown to encode an essential subunit of the cytochrome
b6f complex that is now called
petN (19). Whereas inactivation of petG,
petL, and petN indicate an essential role of
these subunits for the cytochrome b6f
complex, such experiments have not yet been performed with petM. This may be due to the fact that PetM is nuclear
encoded in C. reinhardtii (20) and higher plants (21),
making its deletion more difficult.
Little information is available on the function of PetM, and its
orientation in the membrane remains controversial. Ketcher and Malkin
(17) argue for a lumenal orientation of the C terminus because of its
more positive character. However, de Vitry et al. (20)
concluded from trypsin treatment experiments with thylakoid membranes
that the C terminus had a stromal orientation.
In this report the petM gene of Synechocystis PCC
6803 was deleted. Analysis of the resulting mutant revealed
characteristic differences from WT that suggest that PetM is not
essential for b6f complex activity
but instead plays a regulatory role.
Growth Conditions--
Synechocystis PCC 6803 wild
type and the petM Cloning, DNA Sequencing, and Gene Inactivation--
Cloning was
carried out using standard techniques as described in Ref. 23. Enzymes
used for PCR and cloning were obtained from MBI Fermentas and New
England Biolabs. PCR was done in a Bio-Rad Thermocycler for 30 cycles
under the following conditions: 95 °C for 1 min, 60 °C for 1 min,
and 72 °C for 2 min. The following primers were used for
amplification of the petM gene alone and the petM
gene with flanking regions: CT PetM, TTATTCTTCGCTACCTTGGAGTTT; NT PetM, ATGACCGCTGAAAGCATGTTGGCC; CT PetM+, AGCACCATTTGGGCTTGGGTGTCC; NT PetM+, TTTGAGCGGAATATGGGTGGTGGC. A 1.0-kilobase pair DNA
fragment encoding the petM gene and flanking regions from
Synechocystis PCC6803 was amplified by PCR. The
NheI-restricted DNA fragment was cloned into the
NheI side of the pDMI.1 (24) plasmid, resulting in the
plasmid pPMF. DNA sequencing was performed by MWG Biotech (Ebersberg, Germany).
For Southern hybridization the amplified petM gene was
labeled with digoxygenin using the instructions of the non-radioactive labeling kit (Roche Molecular Biochemicals). Genomic
Synechocystis DNA was prepared according to Ref. 25.
Insertional inactivation of the petM gene in
Synechocystis was carried out according to Ref. 26.
Spectroscopic Techniques--
Absorbance spectra of cell lysates
in the visible range were recorded with a Beckman spectrophotometer as
described in Ref. 27, and fluorescence emission of cells was
recorded with an Aminco Bowman Series 2 fluorimeter as described
in Ref. 39. For fluorescence measurements, cells were diluted in BG11
medium containing 20% glycerol to a final chlorophyll concentration of 5 µg/ml and frozen in liquid nitrogen. Fluorescence induction measurements were carried out as described
previously.2
Single turnover kinetics of cytochrome f and
P700 (the photosynthetic reaction center of
photosystem I) were monitored at room temperature using an
in-house-constructed single beam spectrophotometer as described in Ref.
29. Synechocystis cells were suspended to a final
chlorophyll concentration of 12 µg/ml in BG11 medium. Cytochrome
f and P700 kinetics were determined by analysis
of flash-induced absorbance changes at 551, 556, 561, and 703 nm. Flashes of 6-ms half peak width were provided to both sides of the
sample with a xenon flashlamp (20 millifarad capacitance) filtered with BG635 filters. Cytochrome f redox changes were
calculated from the absorbance difference at 556 nm minus the average
of 551 nm and 561 nm and with an assumed extinction coefficient of 18 mM Data Analysis--
Protein sequence information was analyzed
using the public domain CuraTools protein analysis software (CuraGen).
Similarity searches were performed using the BLAST program provided by
the National Center for Biotechnology Information.
Generation of a petM
According to the Synechocystis genome sequence, an open
reading frame of unknown function (smr0003) is located directly
upstream of the interrupted gene. To check for polar effects on this
possibly cotranscribed gene, an alternative inactivation strategy for
the petM gene was also carried out. The pPMF plasmid was
restricted with BspHI, and the 4-nucleotide overhang was
filled in by T4 polymerase. Religation of the plasmid caused a
frameshift in the petM gene. After insertion of a
chloramphenicol resistance cassette at the BspE I site of
the plasmid, the construct was used for transformation of
Synechocystis. The resulting mutant was checked for complete
segregation by restricting the PCR-amplified petM gene with
BspHI. Only the wild-type gene could be restricted with this
enzyme, because the frameshift introduced into the mutated gene caused
the loss of this restriction site. The completely segregated mutant
showed the same phenotype as the mutant with the interrupted gene in
all measurements carried out. This indicated that either the
interruption of the petM gene did not affect the expression
of this putative gene or that the additional inactivation of this open
reading frame did not yield any phenotype.
Pigment Composition--
Fig. 2
shows absorbance spectra of Synechocystis wild-type and
mutant cells grown under low (20 µmol photons
m
To test for additional effects of the petM disruption on the
photosynthetic pigment composition, fluorescence emission spectra of
whole cells were recorded at 77 K upon excitation of chlorophyll at 435 nm or of phycobilisomes at 580 nm (Fig.
3). Peaks at about 650 and 665 nm can be
attributed to phycobilisome pigments, and the peak at 685 nm can be
attributed to the phycobilisome terminal emitter allophycocyanin
B and the small antenna of PS II. The peak at 695 nm is
associated with CP47 of PS II, and the peak at 725 nm originates from
PS I (31). In the mutant, a decreased amount of phycobiliproteins
relative to chlorophyll is obvious from Fig. 3. Fig. 3, A
and C, illustrates a decreased emission peak at 725 nm in
the mutant, particularly under HL conditions. Because the chlorophyll
content per cell is decreased by up to 20% in the mutant relative to
the wild type (as assessed from the ratio of chlorophyll to
OD730), the amount of PS I apparently is decreased in the
mutant. In addition, because PS I binds about 90% of the total
cellular chlorophyll, and the relative decrease of the emission peak at
725 nm is more than 20%, an increase in the amount of PS II per cell
is also likely.
Electron Transport Activity--
To check the activity of the
remaining PS I complexes, the primary donor P700 of PS I
was oxidized by a single turnover flash, and the re-reduction by
plastocyanin was monitored. As shown in Fig.
4, the kinetics of the wild type
(A) and petM
In contrast to higher plants, the PQ pool of the cyanobacterium
Synechocystis PCC 6803 is mainly reduced in the
dark,2 indicating that the intersystem components
cytochrome f, the Rieske protein, plastocyanin, and
P700 will also be reduced under these conditions.
Photooxidation of P700 by a single saturating xenon flash
is followed within a millisecond by oxidation of intersystem components as P700 is re-reduced. This in turn is followed
by re-reduction of the intersystem components by plastoquinol,
catalyzed by the cytochrome b6f
complex and occurring at a rate governed by the degree of reduction of
the plastoquinone pool. The re-reduction kinetics of cytochrome
f, monitored as described above, is shown in Fig.
5 under aerobic conditions for WT
(A) and the petM Status of the PQ Pool--
The redox state of the PQ pool in the
mutant was monitored by fluorescence induction measurements. This is
possible because the redox state of QA depends on
the redox state of the PQ pool.2 Fig.
6, A-D, shows fluorescence
induction curves of wild-type and petM
This suggests that another component besides the cytochrome
b6f complex is capable of oxidizing
the PQ pool. Fig. 6F shows that a combination of
2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and
pentachlorophenol, an inhibitor of quinol oxidases, yields similar
kinetics of variable fluorescence in the mutant and the wild type,
strongly indicating an increased activity of a quinol oxidase in the
mutant that is capable of keeping the PQ pool in a more oxidized condition.
The Role of PetM in the Cytochrome b6f
Complex--
Previous inactivation experiments of PS II (32), PS I
(33, 34), the NADH dehydrogenase (35), and the terminal oxidases (24)
resulted in fully segregated mutants of Synechocystis PCC 6803 and indicate that these complexes are not essential for the survival of the cells. In contrast, inactivation of genes encoding the
main cytochrome b6f subunits have
failed so far to produce a viable organism (36); this is due to the
absence of a cytochrome bc1 complex in
Synechocystis PCC 6803 and the essential function of the
cytochrome b6f complex in both
respiratory and photosynthetic electron transport. For this
reason, inactivation or site-directed mutagenesis leading to
inactivation of this complex is not possible, in contrast to other
organisms such as the green alga C. reinhardtii, where the
b6f complex is nonessential (37).
In C. reinhardtii deletion of the gene encoding the small
cytochrome b6f subunit PetG (38), as
well as the inactivation of the petL gene (16), resulted in
no, or impaired, photoautotrophic growth; in addition, the amount of
other cytochrome b6f subunits is
strongly reduced. Additionally, a new small subunit of the cytochrome
b6f complex, PetN, was recently
identified, the inactivation of which also led to a photosynthetically
incompetent phenotype (19).
In contrast, petM can be fully deleted in
Synechocystis, resulting in only minor effects on the
cytochrome b6f complex. This is the
first time that a complete deletion of a cytochrome
b6f subunit in
Synechocystis has been described. Inactivation of
petM in Synechocystis did not affect the amount
of assembled b6f complex in the
membranes as quantified from the extent of the cytochrome f
kinetic transients. Under all tested conditions growth is normal (data
not shown, but see Ref. 39), and there is also not a very obvious
phenotype under various light intensities. These results clearly show
that the PetM subunit has no essential role in
b6f electron transfer function in the
cyanobacterium Synechocystis PCC 6803; however, the absence
of this subunit apparently affects the levels of other protein
complexes involved in energy transduction.
Effects on Other Protein Complexes--
Our results clearly show
an impact of PetM on some protein complexes of Synechocystis
that are involved in the electron transport chain. Whereas the amount
of the cytochrome b6f complex is
similar in mutant and wild-type strains, both PS I and phycobilisome
content are reduced in the mutant strain. Furthermore, the re-reduction kinetics of cytochrome f are different under aerobic
conditions; this could result from an altered cytochrome
b6f complex activity or a more
oxidized redox state of the PQ pool. Because the activity of the
complex was the same in the mutant and WT under anaerobic conditions,
the different aerobic re-reduction behavior probably arises from an
increased oxidation state of the PQ pool in the aerobic state.
Furthermore, the fluorescence induction measurements indicate that
electrons can leave the PQ pool of the petM
A strong candidate for this alternative route of plastoquinol oxidation
under aerobic conditions is the cytochrome bd oxidase, especially because pentachlorophenol, a specific inhibitor of the
cytochrome bd oxidase in Synechocystis (25), is
able to restore the mutant fluorescence induction kinetics to resemble that of WT. The comparison with wild-type cells suggests an increased activity of this oxidase in the thylakoid membrane of the
petM Model for a Regulatory Role of the Cytochrome b6f
Complex--
A regulatory function of the redox state of the PQ pool
has been postulated for a long time, and analysis of this phenomenon is
an active field of research. In some cases the redox state of the PQ
pool itself seems to be the signal for further regulatory steps (40),
whereas in other cases the cytochrome
b6f complex seems to be directly
involved in signaling.
It has been shown that the cytochrome
b6f complex can regulate the PS I/PS
II stoichiometry in cyanobacteria (41). Changing light conditions or
cytochrome b6f complex inhibitor
treatment affects the amount of PS I, whereas the content of PS II and
the cytochrome b6f complex remains
constant. These observations are in good agreement with our results and
support the conclusion that the PS I content has changed in the
petM
In addition, the data presented in this work indicate an increased
activity of a quinol oxidase. This could be due either to an activation
of this complex or to accumulation of more quinol oxidase in the
membrane. In the case of PS I it seems already clear that the
cytochrome b6f complex controls the
assembly process of this complex (42).
A working model based on the data presented here is shown in Fig.
7. The cytochrome
b6f complex seems to control the
activity or assembly of the phycobilisomes, of PS I, and of the
cytochrome bd oxidase. The relative stoichiometries of these
complexes, as gained from the presented data, indicate that the
regulatory function of the cytochrome
b6f complex is impaired;
i.e. the inactivation of petM yields a cytochrome
b6f complex with altered regulatory properties. Because the redox activity of the complex seems unaffected, PetM itself must have a role in the accumulation of other protein complexes.
PetM is extremely hydrophobic and of relatively small size. Both facts
argue against a role of PetM in the expression or post-translational modification of other cytochrome b6f
subunits. Hydropathy analysis of PetM suggests an intrinsic protein
with one transmembrane helix.
A new small intrinsic subunit (PetO) of the cytochrome
b6f complex of C. reinhardtii with a possible regulatory function has recently been
identified (28). Because there exists no homologous protein in
Synechocystis and because the function of PetM is completely unknown, it seems possible that this small subunit is involved in a
signal transduction process involving the cytochrome
b6f complex in this cyanobacterium.
The inactivation of the petM gene may effect a signal
transduction cascade with concomitant influence on protein
stoichiometries in the thylakoid membrane of
Synechocystis.
In summary, the presented data indicate that the cytochrome
b6f subunit PetM is not essential for
the redox activity of this complex. Although the mutant strain shows no
apparent phenotype, PetM seems to be important for regulatory circuits
including a quinol oxidase in which the cytochrome
b6f complex may be involved.
mutant as judged by cytochrome
f re-reduction and oxygen evolution rates. In contrast to
the wild type, however, the content of phycobilisomes and
photosystem I as determined from 77 K fluorescence spectra is
reduced in the petM
strain. Furthermore,
whereas under anaerobic conditions the kinetics of cytochrome
f re-reduction are identical, under aerobic conditions these kinetics are slower in the petM
strain.
Fluorescence induction measurements indicate that this is due to an
increased plastoquinol oxidase activity in the mutant, causing the
plastoquinone pool to be in a more oxidized state under aerobic dark
conditions. The finding that the activity of the cytochrome
b6f complex itself is unchanged,
whereas the stoichiometry of other protein complexes has altered,
suggests an involvement of the PetM subunit in regulatory processes
mediated by the cytochrome b6f complex.
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
-helices, respectively. Cytochrome f is a 31-kDa c-type
cytochrome with a covalently bound heme c in the large lumen-exposed
domain; it is anchored by a single C-terminal
-helix in the
membrane. Similarly, the Rieske iron-sulfur protein has a large
hydrophilic lumenal domain attached to a single transmembrane
-helix
at the N terminus.
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
(petM
interruption) mutant strains were grown at 30 °C in BG11 medium (22) aerated with air enriched in CO2 and at light intensities
of 20 or 100 µmol photons m
2
sec
1. For selection of
spectinomycin-resistant mutants, BG11 medium was supplemented with
increasing concentrations of spectinomycin (5-100 µg/ml).
1
cm
1. Redox changes of P700 were
monitored at 703 nm with an assumed extinction coefficient of 64 mM
1
cm
1. To obtain anaerobic conditions, 10 mM glucose, 400 units/ml catalase, and 24 units/ml glucose
oxidase were added, and samples were incubated for 5 min prior to the measurement.
RESULTS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
Strain--
For the generation
of a petM
Synechocystis strain, the
5.7-kilobase pair plasmid pPMF was cleaved with BspHI, which
has a unique site in the petM gene, and the resulting
termini of the plasmid were filled in using T4 DNA polymerase. A
spectinomycin/streptomycin resistance cassette containing the
aadA+ gene was obtained by restricting the
plasmid pHP45
(30) with SmaI and ligated to the
linearized plasmid pPMF. The resulting 7.8-kilobase pair plasmid pPMS
was used for transformation of the Synechocystis wild-type
cells. A completely segregated petM
mutant was
confirmed by PCR after several subcultures in BG11 medium with
increasing amounts of spectinomycin. Fig.
1A shows a fragment of about
100 base pairs carrying the petM gene, which was amplified
from wild-type DNA, whereas a fragment of about 2.2 kilobase pairs was
obtained with DNA from the petM
mutant
corresponding to the size of the petM gene interrupted by
the resistance cassette. No 100-base pair fragment was detectable in
the mutant under various PCR conditions, indicating a complete segregation, which was confirmed by Southern blotting experiments (Fig.
1B).
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Fig. 1.
Analysis of DNA from the wild type and the
petM mutant.
A, PCR products of the petM gene from both
strains; in the case of petM
DNA, only a
fragment corresponding to the interrupted petM gene can be
amplified. B, Southern blot of NcoI-restricted
DNA using the labeled petM gene as a probe. For the
petM
strain only a 2.85-kilobase
fragment is visible, which corresponds to the interrupted
petM gene. MW, molecular weight;
bp, base pairs; SpR, spectinomycin
resistance.
2 sec
1; LL) and
medium high light intensity (100 µmol photons
m
2 sec
1; HL). Under
both conditions, particularly at higher light intensity, the level of
phycobilisomes (absorption peak at 620 nm) is decreased in the
petM
strain. This is reflected in a
yellow-green color of the mutant in comparison with the blue-green
color of the wild type.
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Fig. 2.
Absorbance spectra of wild-type (-) and
petM (- -) strains of
Synechocystis grown under different conditions.
Peaks represent PS I (675 nm), PS II (435 nm), and phycobilisomes (620 nm). petM
cells always show a decreased level
of phycobilisomes, which is stronger under HL conditions (A)
than under LL conditions (B).
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Fig. 3.
77 K fluorescence spectra of
Synechocystis
petM (- -) and
wild-type strain (-) grown under HL (A and
C) and LL (B and D)
conditions with excitation at 580 nm (A and
B) or 435 nm (C and
D). The decreased absorbance at 420 nm can be
correlated with a decreased amount of phycobilisomes. The lower
fluorescence emission at 730 nm additionally indicates a decreased
level of PS I in the cells. The spectra were normalized at 695 nm.
strain (B)
are very similar, indicating that the intrinsic activity of the
remaining PS I complexes is unaffected. For a more quantitative analysis, the amount of P700 was determined as outlined
under "Materials and Methods." The wild type yielded 3.27 mmol of
P700/mol of chlorophyll, and the mutant yielded only
2.48 mmol of P700/mol of chlorophyll; this clearly confirms
the increased PS II:PS I ratio in the petM
strain observed by 77 K fluorescence spectroscopy.
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Fig. 4.
P700 turnover kinetics of PS
1. Wild-type (A) and petM
strains (B) show the same kinetics of
re-reduction. Identical chlorophyll concentrations were used for both
types of cells, which were flashed four times with far red light while
absorbance changes were monitored at 703 nm.
strain
(B). Although both exhibit oxidation followed by
re-reduction on this timescale, a considerable part of cytochrome
f in the petM
mutant remains
oxidized in comparison with the wild type. This effect could be caused
either by a malfunctioning fraction of the cytochrome
b6f complex or by a more oxidized
redox state of the PQ pool in the mutant. Interestingly, under
anaerobic conditions, the re-reduction kinetics of cytochrome
f in the petM
mutant are almost
identical to those of the wild type (Fig. 5, C and
D). This indicates that the aerobic difference arises
because the plastoquinone pool is more oxidized in the mutant, rather than because of any difference in the
b6f function itself. Moreover, quantification of the extent of the cytochrome f transient
shows that the content of the cytochrome
b6f complex in the mutant is similar
to that of the wild type (Fig. 5).
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Fig. 5.
Transient kinetics of the cytochrome
f turnover in wild-type (A and
C) and
petM (B
and D) cells under aerobic (A
and B) and anaerobic conditions (C
and D). Cells were illuminated with
far red light, and the cytochrome f re-reduction was
determined as indicated under "Materials and Methods."
Abs, absorption.
cells
grown under LL and HL conditions and in the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an
inhibitor of cytochrome b6f
oxidoreductase activity. For comparison, the maximum fluorescence yield
of cells from different strains was measured by blocking the
reoxidation of QA
with
3-(3,4-dichlorophenyl)-1,1-dimethylurea at the QB
site of PS II. Interestingly, the variable fluorescence
(FV) in the petM
strain
(Fig. 6D) did not reach the maximal level obtained by the
addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, in contrast to the
behavior of the wild-type strain (Fig. 6B). This effect is
even stronger under HL (Fig. 6, A and C),
although under these conditions WT also does not reach the maximum
level. Even an enhanced illumination of 4 s could not help the
mutant strain to reach the maximum fluorescence yield (Fig.
6E), leaving the PQ pool partly oxidized.
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Fig. 6.
Fluorescence induction curves from wild-type
(WT) (A and B) and
petM
cells (C and
D) grown under HL (A and
C) and LL (B and D)
conditions after blocking the cytochrome
b6f complex
(A-E) and the cytochrome bd oxidase
(F). Illumination for 1 s (A-D)
or 4 s (E and F). E and
F have an enlarged ordinate scale, starting at the
FO level, to show the variation of
FV more clearly. The upper
curve in each case was always recorded in the presence of 10 µM 3-(3,4-dichlorophenyl)-1,1-dimethylurea.
DBMIB,
2,5-dibromo-3-methyl-6- isopropyl-p-benzoquinone;
PCP, pentachlorophenol.
DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
mutant even when the cytochrome b6f
complex is inhibited.
mutant. Whereas the spectroscopic
data indicate that the subunit PetM is not required for the stability,
assembly, or redox activity of the cytochrome
b6f complex, changes in the activity
of other protein complexes in the membrane imply a specific regulatory role of the cytochrome b6f complex in
general and the PetM subunit in particular.
mutant without, however, having any
effect on its re-reduction kinetics.
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Fig. 7.
Model for a regulatory function of the
cytochrome b6f complex. Small arrows
indicate electron pathways, and large arrows indicate
possible regulatory effects of the cytochrome
b6f complex on various protein
complexes. PBS, phycobilisome;
PQH2, plastoquinol; cyt, cytochrome;
Pc, plastocyanine.
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ACKNOWLEDGEMENTS |
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We thank Dr. C. Mullineaux and Dr. W. Vermaas for stimulating discussions and critical reading of the manuscript. The excellent technical assistance of U. Altenfeld and J. Ramsey is gratefully acknowledged.
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FOOTNOTES |
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* This work was supported by the Deutsche Forschungsgemeinschaft (SFB 480) and the Human Frontier Science Program (to D. S. and M. R.).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.
¶ To whom correspondence should be addressed. E-mail: Matthias.Roegner@ruhr-uni-bochum.de.
Published, JBC Papers in Press, February 13, 2001, DOI 10.1074/jbc.M009503200
2 S. Berry, D. Schneider, W. F. J. Vermaas, and M. Rögner, submitted for publication.
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ABBREVIATIONS |
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The abbreviations used are: PS, photosystem; WT, wild type; PCR, polymerase chain reaction; LL, low light; HL, high light; PQ, plastoquinone.
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