BACKGROUND
the assembly, composition and maintenance of photosynthetic pigment-protein complexes are regulated by many different factors including light quality, light intensity, temperature, water availability and nutrient status  <cit> .
microorganisms have diverse mechanisms for sensing and acclimating to environmental parameters.
in bacteria, two-component signal transduction pathways form a major part of the signalling machinery, mediating adaptive responses to a broad range of environmental stimuli  <cit> .
in response to such stimuli, a sensor kinase autophosphorylates an internal histidine residue, and then transfers the phosphate to a conserved aspartate residue on the receiver domain of its cognate response regulator.
the phosphorylation of the response regulator often leads to changes in gene transcription  <cit> .
response regulators are generally phosphorylated by their cognate sensor kinases, but phosphorylation can also be mediated through low molecular mass phosphate donors, such as acetyl phosphate, phosphoramidate  and carbamyl phosphate  <cit> .
the pta gene encodes phosphotransacetylase which catalyses the production of acetyl phosphate, the activated acetate intermediate that occurs during conversion of acetyl-coa to acetate  <cit> .
chamnongpol & groisman  <cit>  identified acetyl phosphate as the phosphate donor for a mutant version of the response regulator phop  and showed that inactivation of the pta gene abolished phop*-mediated transcription.
most response regulators can be phosphorylated by acetyl phosphate in vitro  <cit> , but at physiological concentration this may not be significant depending on the km for each individual response regulator for acetyl phosphate.
it may be that acetyl phosphate maintains a low level of phosphorylation for most response regulators.
the pta gene has been identified in synechocystis  <dig>  and crocosphaera watsonii by homology to other bacterial pta sequences , however pta has not been identified in the genome sequence of the other cyanobacteria.
the gene dspa  codes for a putative sensory histidine kinase that is conserved in plastids and cyanobacteria.
the deduced amino acid sequence of dspa contains  <dig> amino acids in synechocystis sp.
pcc  <dig> .
dspa has a hamp  <cit>  and a pas sensor domain  <cit> .
we have also identified a possible additional sensory domain in ycf <dig>  which will be discussed below.
all cyanobacterial genomes sequenced to date contain an orf encoding a ycf <dig> orthologue and ycf <dig> is also the only histidine kinase gene present in the plastid genomes of the red algae porphyra purpurea, gracilaria tenuistipitata and cyanidium caldarium  <cit> .
this suggests that it performs a key role in the regulation of the level of photosynthetic pigment-protein complexes.
the probable orthologue for ycf <dig> in the cyanobacterium synechocystis  <dig> is an orf designated sll <dig> in the cyanobase genome database  which has also been called hik <dig>  <cit>  and dspa  <cit> .
it was identified as a factor that confers resistance to a variety of herbicides and was suggested to be a histidine kinase involved in chemical sensing  <cit> .
it was found to be involved in gene expression , demonstrating characteristics of a sensor that can detect a decrease in membrane fluidity, and is involved in the perception and transduction of salt stress and low temperature signals in synechocystis  <dig>  <cit> .
deletion of this open reading frame leads to global changes in gene expression  <cit>  and in addition a potential link has been identified between hik <dig> and rre <dig> .
in another cyanobacterium, synechococcus sp.
pcc <dig> , the probable orthologue for ycf <dig> was shown to regulate phycobilisome degradation during nutrient starvation and in high light.
the gene was therefore designated nbls   <cit> .
dspa is also important for the survival of synechocystis  <dig> when grown in high-light  <cit> .
one possible cognate response regulator  has been identified in synechococcus  <dig> for nbls, however, no orthologue for nblr has been identified in synechocystis  <dig>  and it is likely that nbls also interacts with other response regulators  <cit> .
nbls has been proposed to integrate redox and light signals and may influence other signalling pathways involved in acclimation responses  <cit> .
thus it appears that the orthologues of dspa in other cyanobacteria have different but overlapping functions.
a role for the sll <dig> gene product in controlling the degradation of the photosynthetic apparatus during nutrient stress has not yet been demonstrated in synechocystis  <dig>  we have constructed an insertional deletion mutant in this orf, which we will refer to as dspa.
we show that the inactivation of dspa has little phenotypic effect under nitrogen starvation, unless the mutation is combined with deletion of the pta gene.
in the double mutant there is a dramatic effect on the degradation of chlorophyll-protein complexes under nitrogen starvation, which is not seen in either of the single mutants, however phycobilisomes are degraded, though not to the same level as in wild type.
we conclude that acetyl phosphate masks dspa signalling under our laboratory conditions.
the results indicate that the degradation of photosynthetic complexes during nutrient stress is controlled by multiple inputs, including acetyl phosphate level.
RESULTS
analysis of cyanobacterial and plastid deduced ycf <dig> protein sequences
the deduced amino acid sequences for ycf <dig> from  <dig> available cyanobacterial annotated genome sequences and the red algal species porphyra purpurea, gracilaria tenuistipitata and cyanidium caldarium were analysed by pfam and smart.
this identified a hamp, pas , hiska and hatpase_c domain .
smart and interpro also identified two potential transmembrane helical regions upstream of the hamp domain .
the identification of transmembrane sequences by smart is with an accuracy of 97–98%  <cit> .
because the  <dig> amino acid region that is flanked by the two transmembrane regions is highly conserved in all  <dig> cyanobacterial sequences and porphyra  and it is adjacent and upstream of a hamp domain , we think it probably functions as a periplasmic sensor.
we have not been able to detect homologous sequences in any other protein sequence, suggesting that it is unique to ycf <dig> .
the pas sensor domain may bind a redox sensitive cofactor  <cit>  that could potentially report on the metabolic state of the cell.
phylogenetic analysis  and clustalw alignments of these sequences and the alignment scores indicate that all these proteins  are probably orthologues .
this is supported by tu et al.,  <cit>  who were able to rescue their dspa null mutant in synechocystis  <dig> with the nbls gene from synechococcus  <dig>
phenotype of the pta, dspa and pta/dspa mutants under nutrient-replete conditions
mutants were routinely characterised by whole cell absorption spectra, which can be used to give an estimate of the amount of chlorophyll and phycocyanin per cell  <cit> .
mutants were further characterised by  <dig> k fluorescence emission spectra.
these spectra give an indication of changes in the psii:psi ratio  <cit> .
when cells were grown under low light  the absorption spectra were similar, indicating little difference in the chlorophyll and phycocyanin content of the cells .
when cells were grown under higher light , only slight differences in absorption spectra and chl/pc ratios were seen .
however low-temperature fluorescence spectra recorded with chlorophyll excitation revealed changes in the ratio of psii/psi .
these spectra show peaks at  <dig> nm and  <dig> nm  and  <dig> nm   <cit> .
spectra have been normalised to the psi fluorescence emission, as their absolute amplitudes are unreliable  <cit> .
when cells were grown at  <dig> μmol m- <dig> s- <dig>  the dspa and pta single mutants show a lower psii/psi ratio than the wild-type, and this difference was more pronounced in the pta/dspa double mutant .
a similar pattern was seen in cells grown at  <dig> μmol m- <dig> s- <dig>  but the differences were less pronounced .
phenotype of the pta, dspa and pta/dspa mutants after nitrogen starvation
an nbls mutant of synechococcus  <dig> shows a pronounced phenotype under nitrogen starvation.
in the wild-type, cells gradually lose their photosynthetic pigments under these conditions.
however, an nbls mutant is bleached to a much lesser extent, retaining its phycobilisomes in particular  <cit> .
we therefore looked at the pigmentation of our synechocystis  <dig> mutants during growth in modified bg <dig> medium supplemented with glucose , but containing no nano <dig>
after three days nitrogen starvation in moderate light , the wild-type and the dspa and pta single mutants were almost completely bleached .
we could detect no significant differences  in the rate of bleaching during nitrogen starvation in the three strains .
by contrast, the pta/dspa double mutant retained significant levels of pigment .
both the chlorophyll and phycocyanin per cell dropped somewhat compared to the start of the treatment, but loss of phycocyanin was much greater .
thus only the pta/dspa double mutant shows a 'non-bleaching like' phenotype: chlorophyll concentration decreases by a factor of about two and that of phycocyanin by about five.
figure  <dig> shows low-temperature fluorescence emission spectra for the pta/dspa double mutant grown after  <dig> days nitrogen starvation.
whether with or without nitrate, it exhibits a similar spectrum meaning that the starved cells retain both psi and psii.
the starved pta-/dspa- cells also retained significant levels of oxygen evolution .
thus the pta/dspa double mutant retains its photosynthetic apparatus during nitrogen starvation, in contrast to the wild-type and both single mutants, where nearly all pigment is lost.
discussion
a potential new sensor domain in dspa/ycf26
ycf <dig> may have a periplasmic sensor domain, which could play an important role in sensing stress either directly or indirectly .
the putative periplasmic sensor and pas domain may combine to enable ycf <dig> to sense a wide variety of stress conditions .
it remains to be determined whether dspa senses any of these signals directly or via other proteins or phosphorelays and the precise way in which these sensor domains interact.
this apparent central role of ycf <dig> in the regulation of gene expression in response to a number of different stresses is supported by its universal occurrence in the  <dig> available cyanobacterial genome sequences, particularly the  <dig> prochlorococcus species which only have 4– <dig> histidine kinases in their genome .
roles of dspa and acetyl phosphate in cell signalling
we have constructed synechocystis  <dig> mutants deficient in the pta gene coding for phosphotransacetylase and the dspa gene coding for the dspa sensory histidine kinase , and a double mutant deficient in both genes.
we have used the mutants to examine the possible roles of dspa and acetyl phosphate in controlling cell responses to nitrogen starvation.
we find strong evidence that dspa and acetyl phosphate act cooperatively as signalling inputs into pathways controlling photosystem stoichiometry under nutrient-replete conditions, and photosystem degradation during nitrogen starvation.
under nutrient-replete conditions, the dspa and pta single mutants both show a decreased psii/psi ratio .
the psii/psi ratio becomes even lower in the pta/dspa double mutant, indicating additive effects of the two mutations.
since acetyl phosphate is known to act as a phosphodonor to response regulators  <cit> , we propose that psii/psi ratio is influenced by gene expression controlled by one or more response regulators  whose phosphorylation level is controlled by phosphotransfer both from dspa and from acetyl phosphate.
this scheme  is illustrated in figure  <dig>
the effects seen under nitrogen starvation are even more striking.
after  <dig> days' nitrogen starvation under moderate light, wild-type cells are virtually devoid of pigment, indicating almost complete loss of the phycobilisomes and the chlorophyll-protein complexes .
in this case, the dspa and pta single mutants both behaved in the same way as the wild-type, but the pta/dspa double mutant retained a significant proportion of its photosynthetic complexes, in particular the chlorophyll-protein complexes.
therefore the pta/dspa double mutant is clearly deficient in a signalling pathway required to initiate degradation of these complexes under these conditions.
again, we suggest that this pathway involves gene expression controlled by one or more response regulators whose phosphorylation level is determined by phosphotransfer from both dspa and acetyl phosphate .
in this case the double mutation is needed to produce a detectable phenotypic effect, suggesting that either dspa or acetyl phosphate alone are sufficient to maintain high enough levels of phosphorylation of the relevant response regulator .
the fact that the phenotypic difference is seen only in the double mutant very strongly suggests that dspa and acetyl phosphate are acting as phosphodonors to the same response regulator.
again, it appears that a high level of response regulator phosphorylation promotes the degradation of the photosynthetic complexes.
acetyl phosphate as a potential global regulator of gene expression in synechocystis 6803
in e. coli the concentration of acetyl phosphate has been shown to be dependent on the metabolic state of the cell as well as the growth phase, carbon source, ph and temperature  <cit> , therefore acetyl phosphate could potentially serve as a global sensor of the physiological state of the cell in e. coli.
acetyl phosphate is known to phosphorylate several rr's, including chey, nri, phob and ompr  <cit> in vivo, but its ability to phosphorylate response regulators in vivo depends on the km of the response regulator for acetyl phosphate, which in many cases is too high to allow phosphorylation  <cit> .
in the four systems studied in one investigation of crosstalk between two component systems, one, the uhp system was shown to be activated by acetyl phosphate when e. coli is grown on pyruvate  <cit> .
there have been no comparable studies in cyanobacteria, but it seems likely that the level of acetyl phosphate would change in response to physiological stress, in particular nutrient deprivation.
if this is the case, it seems likely that the km of at least some response regulators could be tuned to the physiological concentrations of acetyl phosphate in synechocystis  <dig>  allowing the level of acetyl phosphate to be part of the signalling process.
the level of pigment complexes in the pta mutant was close to normal in nutrient replete conditions , indicating that the level of acetyl phosphate is not having an effect on overall expression of photosynthetic gene expression via a number of response regulators, but we cannot rule out that the cumulative effect of pta- and dspa- that we observe in nutrient depleted conditions may be influenced by a reduced phosphorylation of response regulator other than those phosphorylated by dspa.
role of dspa in initiating degradation of photosynthetic complexes during nutrient stress in synechocystis 6803
nbls  was originally identified in synechococcus  <dig> as a factor important for initiating the degradation of the photosynthetic complexes  during nitrogen starvation  <cit> .
in synechocystis  <dig> there is strong evidence for dspa as a global regulator controlling the expression of many genes involved in photosynthesis, carbon metabolism and stress responses  <cit> .
however, dspa mutations in synechocystis  <dig> have not previously been shown to induce a non-bleaching phenotype like that seen in synechococcus  <dig>  here we have shown that such a phenotype can be seen in the dspa mutant of synechocystis  <dig>  provided that the pta gene is also deleted.
the phenotype is still not identical to that seen in synechococcus  <dig>  however, since in synechocystis  <dig> it is not only the degradation of the phycobilisomes which is affected but also the degradation of chlorophyll-protein complexes .
CONCLUSIONS
it is clear that, under our conditions, dspa is a sensor for nitrogen starvation in association with the level of acetyl phosphate, leading to photosystem degradation, since this response occurs similarly in the wild-type and in the dspa single mutant .
however, it is unlikely to be coincidental that non-bleaching phenotypes are associated with nbls/dspa mutations both in synechococcus  <dig> and in synechocystis  <dig>  we suggest that there must be other, as yet undetermined, conditions in which acetyl phosphate levels in wild-type cells are low.
under such conditions, dspa activity will become a physiologically important factor controlling the degradation of the photosynthetic complexes.
the presence of pta in synechocystis  <dig> may reflect its more flexible metabolic lifestyle compared to other cyanobacteria.

 the assembly, composition and maintenance of photosynthetic pigment-protein complexes are regulated by many different factors including light quality, light intensity, temperature, water availability and nutrient status. microorganisms have diverse mechanisms for sensing and acclimating to environmental parameters. in bacteria, two-component signal transduction pathways form a major part of the signalling machinery, mediating adaptive responses to a broad range of environmental stimuli. in response to such stimuli, a sensor kinase autophosphorylates an internal histidine residue, and then transfers the phosphate to a conserved aspartate residue on the receiver domain of its cognate response regulator. the phosphorylation of the response regulator often leads to changes in gene transcription. response regulators are generally phosphorylated by their cognate sensor kinases, but phosphorylation can also be mediated through low molecular mass phosphate donors, such as acetyl phosphate, phosphoramidate and carbamyl phosphate. the pta gene encodes phosphotransacetylase which catalyses the production of acetyl phosphate, the activated acetate intermediate that occurs during conversion of acetyl-coa to acetate. chamnongpol & groisman identified acetyl phosphate as the phosphate donor for a mutant version of the response regulator phop and showed that inactivation of the pta gene abolished phop*-mediated transcription. most response regulators can be phosphorylated by acetyl phosphate in vitro, but at physiological concentration this may not be significant depending on the km for each individual response regulator for acetyl phosphate. it may be that acetyl phosphate maintains a low level of phosphorylation for most response regulators. the pta gene has been identified in synechocystis and crocosphaera watsonii by homology to other bacterial pta sequences, however pta has not been identified in the genome sequence of the other cyanobacteria. the gene dspa codes for a putative sensory histidine kinase that is conserved in plastids and cyanobacteria. the deduced amino acid sequence of dspa contains amino acids in synechocystis sp. pcc. dspa has a hamp and a pas sensor domain. we have also identified a possible additional sensory domain in ycf which will be discussed below. all cyanobacterial genomes sequenced to date contain an orf encoding a ycf orthologue and ycf is also the only histidine kinase gene present in the plastid genomes of the red algae porphyra purpurea, gracilaria tenuistipitata and cyanidium caldarium. this suggests that it performs a key role in the regulation of the level of photosynthetic pigment-protein complexes. the probable orthologue for ycf in the cyanobacterium synechocystis is an orf designated sll in the cyanobase genome database which has also been called hik and dspa. it was identified as a factor that confers resistance to a variety of herbicides and was suggested to be a histidine kinase involved in chemical sensing. it was found to be involved in gene expression, demonstrating characteristics of a sensor that can detect a decrease in membrane fluidity, and is involved in the perception and transduction of salt stress and low temperature signals in synechocystis. deletion of this open reading frame leads to global changes in gene expression and in addition a potential link has been identified between hik and rre. in another cyanobacterium, synechococcus sp. pcc, the probable orthologue for ycf was shown to regulate phycobilisome degradation during nutrient starvation and in high light. the gene was therefore designated nbls. dspa is also important for the survival of synechocystis when grown in high-light. one possible cognate response regulator has been identified in synechococcus for nbls, however, no orthologue for nblr has been identified in synechocystis and it is likely that nbls also interacts with other response regulators. nbls has been proposed to integrate redox and light signals and may influence other signalling pathways involved in acclimation responses. thus it appears that the orthologues of dspa in other cyanobacteria have different but overlapping functions. a role for the sll gene product in controlling the degradation of the photosynthetic apparatus during nutrient stress has not yet been demonstrated in synechocystis we have constructed an insertional deletion mutant in this orf, which we will refer to as dspa. we show that the inactivation of dspa has little phenotypic effect under nitrogen starvation, unless the mutation is combined with deletion of the pta gene. in the double mutant there is a dramatic effect on the degradation of chlorophyll-protein complexes under nitrogen starvation, which is not seen in either of the single mutants, however phycobilisomes are degraded, though not to the same level as in wild type. we conclude that acetyl phosphate masks dspa signalling under our laboratory conditions. the results indicate that the degradation of photosynthetic complexes during nutrient stress is controlled by multiple inputs, including acetyl phosphate level.  analysis of cyanobacterial and plastid deduced ycf protein sequences the deduced amino acid sequences for ycf from available cyanobacterial annotated genome sequences and the red algal species porphyra purpurea, gracilaria tenuistipitata and cyanidium caldarium were analysed by pfam and smart. this identified a hamp, pas, hiska and hatpase_c domain. smart and interpro also identified two potential transmembrane helical regions upstream of the hamp domain. the identification of transmembrane sequences by smart is with an accuracy of 97–98%. because the amino acid region that is flanked by the two transmembrane regions is highly conserved in all cyanobacterial sequences and porphyra and it is adjacent and upstream of a hamp domain, we think it probably functions as a periplasmic sensor. we have not been able to detect homologous sequences in any other protein sequence, suggesting that it is unique to ycf. the pas sensor domain may bind a redox sensitive cofactor that could potentially report on the metabolic state of the cell. phylogenetic analysis and clustalw alignments of these sequences and the alignment scores indicate that all these proteins are probably orthologues. this is supported by tu et al., who were able to rescue their dspa null mutant in synechocystis with the nbls gene from synechococcus phenotype of the pta, dspa and pta/dspa mutants under nutrient-replete conditions mutants were routinely characterised by whole cell absorption spectra, which can be used to give an estimate of the amount of chlorophyll and phycocyanin per cell. mutants were further characterised by k fluorescence emission spectra. these spectra give an indication of changes in the psii:psi ratio. when cells were grown under low light the absorption spectra were similar, indicating little difference in the chlorophyll and phycocyanin content of the cells. when cells were grown under higher light, only slight differences in absorption spectra and chl/pc ratios were seen. however low-temperature fluorescence spectra recorded with chlorophyll excitation revealed changes in the ratio of psii/psi. these spectra show peaks at nm and nm and nm. spectra have been normalised to the psi fluorescence emission, as their absolute amplitudes are unreliable. when cells were grown at μmol m- s- the dspa and pta single mutants show a lower psii/psi ratio than the wild-type, and this difference was more pronounced in the pta/dspa double mutant. a similar pattern was seen in cells grown at μmol m- s- but the differences were less pronounced. phenotype of the pta, dspa and pta/dspa mutants after nitrogen starvation an nbls mutant of synechococcus shows a pronounced phenotype under nitrogen starvation. in the wild-type, cells gradually lose their photosynthetic pigments under these conditions. however, an nbls mutant is bleached to a much lesser extent, retaining its phycobilisomes in particular. we therefore looked at the pigmentation of our synechocystis mutants during growth in modified bg medium supplemented with glucose, but containing no nano after three days nitrogen starvation in moderate light, the wild-type and the dspa and pta single mutants were almost completely bleached. we could detect no significant differences in the rate of bleaching during nitrogen starvation in the three strains. by contrast, the pta/dspa double mutant retained significant levels of pigment. both the chlorophyll and phycocyanin per cell dropped somewhat compared to the start of the treatment, but loss of phycocyanin was much greater. thus only the pta/dspa double mutant shows a'non-bleaching like' phenotype: chlorophyll concentration decreases by a factor of about two and that of phycocyanin by about five. figure shows low-temperature fluorescence emission spectra for the pta/dspa double mutant grown after days nitrogen starvation. whether with or without nitrate, it exhibits a similar spectrum meaning that the starved cells retain both psi and psii. the starved pta-/dspa- cells also retained significant levels of oxygen evolution. thus the pta/dspa double mutant retains its photosynthetic apparatus during nitrogen starvation, in contrast to the wild-type and both single mutants, where nearly all pigment is lost. discussion a potential new sensor domain in dspa/ycf26 ycf may have a periplasmic sensor domain, which could play an important role in sensing stress either directly or indirectly. the putative periplasmic sensor and pas domain may combine to enable ycf to sense a wide variety of stress conditions. it remains to be determined whether dspa senses any of these signals directly or via other proteins or phosphorelays and the precise way in which these sensor domains interact. this apparent central role of ycf in the regulation of gene expression in response to a number of different stresses is supported by its universal occurrence in the available cyanobacterial genome sequences, particularly the prochlorococcus species which only have 4– histidine kinases in their genome. roles of dspa and acetyl phosphate in cell signalling we have constructed synechocystis mutants deficient in the pta gene coding for phosphotransacetylase and the dspa gene coding for the dspa sensory histidine kinase, and a double mutant deficient in both genes. we have used the mutants to examine the possible roles of dspa and acetyl phosphate in controlling cell responses to nitrogen starvation. we find strong evidence that dspa and acetyl phosphate act cooperatively as signalling inputs into pathways controlling photosystem stoichiometry under nutrient-replete conditions, and photosystem degradation during nitrogen starvation. under nutrient-replete conditions, the dspa and pta single mutants both show a decreased psii/psi ratio. the psii/psi ratio becomes even lower in the pta/dspa double mutant, indicating additive effects of the two mutations. since acetyl phosphate is known to act as a phosphodonor to response regulators, we propose that psii/psi ratio is influenced by gene expression controlled by one or more response regulators whose phosphorylation level is controlled by phosphotransfer both from dspa and from acetyl phosphate. this scheme is illustrated in figure the effects seen under nitrogen starvation are even more striking. after days' nitrogen starvation under moderate light, wild-type cells are virtually devoid of pigment, indicating almost complete loss of the phycobilisomes and the chlorophyll-protein complexes. in this case, the dspa and pta single mutants both behaved in the same way as the wild-type, but the pta/dspa double mutant retained a significant proportion of its photosynthetic complexes, in particular the chlorophyll-protein complexes. therefore the pta/dspa double mutant is clearly deficient in a signalling pathway required to initiate degradation of these complexes under these conditions. again, we suggest that this pathway involves gene expression controlled by one or more response regulators whose phosphorylation level is determined by phosphotransfer from both dspa and acetyl phosphate. in this case the double mutation is needed to produce a detectable phenotypic effect, suggesting that either dspa or acetyl phosphate alone are sufficient to maintain high enough levels of phosphorylation of the relevant response regulator. the fact that the phenotypic difference is seen only in the double mutant very strongly suggests that dspa and acetyl phosphate are acting as phosphodonors to the same response regulator. again, it appears that a high level of response regulator phosphorylation promotes the degradation of the photosynthetic complexes. acetyl phosphate as a potential global regulator of gene expression in synechocystis 6803 in e. coli the concentration of acetyl phosphate has been shown to be dependent on the metabolic state of the cell as well as the growth phase, carbon source, ph and temperature, therefore acetyl phosphate could potentially serve as a global sensor of the physiological state of the cell in e. coli. acetyl phosphate is known to phosphorylate several rr's, including chey, nri, phob and ompr in vivo, but its ability to phosphorylate response regulators in vivo depends on the km of the response regulator for acetyl phosphate, which in many cases is too high to allow phosphorylation. in the four systems studied in one investigation of crosstalk between two component systems, one, the uhp system was shown to be activated by acetyl phosphate when e. coli is grown on pyruvate. there have been no comparable studies in cyanobacteria, but it seems likely that the level of acetyl phosphate would change in response to physiological stress, in particular nutrient deprivation. if this is the case, it seems likely that the km of at least some response regulators could be tuned to the physiological concentrations of acetyl phosphate in synechocystis allowing the level of acetyl phosphate to be part of the signalling process. the level of pigment complexes in the pta mutant was close to normal in nutrient replete conditions, indicating that the level of acetyl phosphate is not having an effect on overall expression of photosynthetic gene expression via a number of response regulators, but we cannot rule out that the cumulative effect of pta- and dspa- that we observe in nutrient depleted conditions may be influenced by a reduced phosphorylation of response regulator other than those phosphorylated by dspa. role of dspa in initiating degradation of photosynthetic complexes during nutrient stress in synechocystis 6803 nbls was originally identified in synechococcus as a factor important for initiating the degradation of the photosynthetic complexes during nitrogen starvation. in synechocystis there is strong evidence for dspa as a global regulator controlling the expression of many genes involved in photosynthesis, carbon metabolism and stress responses. however, dspa mutations in synechocystis have not previously been shown to induce a non-bleaching phenotype like that seen in synechococcus here we have shown that such a phenotype can be seen in the dspa mutant of synechocystis provided that the pta gene is also deleted. the phenotype is still not identical to that seen in synechococcus however, since in synechocystis it is not only the degradation of the phycobilisomes which is affected but also the degradation of chlorophyll-protein complexes.  it is clear that, under our conditions, dspa is a sensor for nitrogen starvation in association with the level of acetyl phosphate, leading to photosystem degradation, since this response occurs similarly in the wild-type and in the dspa single mutant. however, it is unlikely to be coincidental that non-bleaching phenotypes are associated with nbls/dspa mutations both in synechococcus and in synechocystis we suggest that there must be other, as yet undetermined, conditions in which acetyl phosphate levels in wild-type cells are low. under such conditions, dspa activity will become a physiologically important factor controlling the degradation of the photosynthetic complexes. the presence of pta in synechocystis may reflect its more flexible metabolic lifestyle compared to other cyanobacteria. 
we constructed an insertional mutant of dspa in synechocystis we found little phenotypic effect during nitrogen starvation. under nitrogen starvation, the pta/dspa double mutant degrades its phycobilisomes less than the wild type and still has about half of its chlorophyll-protein complexes. our data indicates that acetyl-phosphate-dependent phosphorylation of response regulator overlaps with dspa-dependent signalling of the degradation of chlorophyll-protein complexes in synechocystis the dspa gene, coding for a putative sensory histidine kinase, is conserved in plastids and cyanobacteria. however, when the mutation was combined with deletion of the pta gene coding for phosphotransacetylase, a more significant phenotype was observed.
 the dspa gene, coding for a putative sensory histidine kinase, is conserved in plastids and cyanobacteria. it has been linked with a number of different stress responses in cyanobacteria.  we constructed an insertional mutant of dspa in synechocystis we found little phenotypic effect during nitrogen starvation. however, when the mutation was combined with deletion of the pta gene coding for phosphotransacetylase, a more significant phenotype was observed. under nitrogen starvation, the pta/dspa double mutant degrades its phycobilisomes less than the wild type and still has about half of its chlorophyll-protein complexes.  our data indicates that acetyl-phosphate-dependent phosphorylation of response regulator overlaps with dspa-dependent signalling of the degradation of chlorophyll-protein complexes in synechocystis 