Effects of light deprivation on RNA synthesis, accumulation of guanosine 3'(2')-diphosphate 5'-diphosphate, and protein synthesis in heat-shocked Synechococcus sp. strain PCC 6301, a cyanobacterium

ppGpp accumulation reduces the expression of the global nitrogen homeostasis-modulating NtcA regulon by affecting 2-oxoglutarate levels

The cyanobacterium Synechococcus elongatus PCC 7942 accumulates alarmone guanosine tetraphosphate (ppGpp) under stress conditions, such as darkness. A previous study observed that artificial ppGpp accumulation under photosynthetic conditions led to the downregulation of genes involved in the nitrogen assimilation system, which is activated by the global nitrogen regulator NtcA, suggesting that ppGpp regulates NtcA activity. However, the details of this mechanism have not been elucidated. Here, we investigate the metabolic responses associated with ppGpp accumulation by heterologous expression of the ppGpp synthetase RelQ. The pool size of 2-oxoglutarate (2-OG), which activates NtcA, is significantly decreased upon ppGpp accumulation. De novo 13C-labeled CO2 assimilation into the Calvin-Benson-Bassham cycle and glycolytic intermediates continues irrespective of ppGpp accumulation, whereas the labeling of 2-OG is significantly decreased under ppGpp accumulation. The low 2-OG levels in the RelQ overexpression cells could be because of the inhibition of metabolic enzymes, including aconitase, which are responsible for 2-OG biosynthesis. We propose a metabolic rearrangement by ppGpp accumulation, which negatively regulates 2-OG levels to maintain carbon and nitrogen balance.

ppGpp Controls Global Gene Expression in Light and in Darkness in S. elongatus

The bacterial and plant stringent response involves production of the signaling molecules guanosine tetraphosphate and guanosine pentaphosphate ((p)ppGpp), leading to global reorganization of gene expression. The function of the stringent response has been well characterized in stress conditions, but its regulatory role during unstressed growth is less studied. Here, we demonstrate that (p)ppGpp-deficient strains of S. elongatus have globally deregulated biosynthetic capacity, with increased transcription rate, translation rate, and cell size in unstressed conditions in light and impaired viability in darkness. Synthetic restoration of basal guanosine tetraphosphate (ppGpp) levels is sufficient to recover transcriptional balance and appropriate cell size in light and to rescue viability in light/dark conditions, but it is insufficient to enable efficient dark-induced transcriptional shutdown. Our work underscores the importance of basal ppGpp signaling for regulation of cyanobacterial physiology in the absence of stress and for viability in energy-limiting conditions, highlighting that basal (p)ppGpp level is essential in cyanobacteria in the environmental light/dark cycle.

The stringent response regulates adaptation to darkness in the cyanobacterium Synechococcus elongatus

The cyanobacterium Synechococcus elongatus relies upon photosynthesis to drive metabolism and growth. During darkness, Synechococcus stops growing, derives energy from its glycogen stores, and greatly decreases rates of macromolecular synthesis via unknown mechanisms. Here, we show that the stringent response, a stress response pathway whose genes are conserved across bacteria and plant plastids, contributes to this dark adaptation. Levels of the stringent response alarmone guanosine 3'-diphosphate 5'-diphosphate (ppGpp) rise after a shift from light to dark, indicating that darkness triggers the same response in cyanobacteria as starvation in heterotrophic bacteria. High levels of ppGpp are sufficient to stop growth and dramatically alter many aspects of cellular physiology, including levels of photosynthetic pigments and polyphosphate, DNA content, and the rate of translation. Cells unable to synthesize ppGpp display pronounced growth defects after exposure to darkness. The stringent response regulates expression of a number of genes in Synechococcus, including ribosomal hibernation promoting factor (hpf), which causes ribosomes to dimerize in the dark and may contribute to decreased translation. Although the metabolism of Synechococcus differentiates it from other model bacterial systems, the logic of the stringent response remains remarkably conserved, while at the same time having adapted to the unique stresses of the photosynthetic lifestyle.

Survival strategies in the aquatic and terrestrial world: the impact of second messengers on cyanobacterial processes

Second messengers are intracellular substances regulated by specific external stimuli globally known as first messengers. Cells rely on second messengers to generate rapid responses to environmental changes and the importance of their roles is becoming increasingly realized in cellular signaling research. Cyanobacteria are photooxygenic bacteria that inhabit most of Earth's environments. The ability of cyanobacteria to survive in ecologically diverse habitats is due to their capacity to adapt and respond to environmental changes. This article reviews known second messenger-controlled physiological processes in cyanobacteria. Second messengers used in these systems include the element calcium (Ca2+), nucleotide-based guanosine tetraphosphate or pentaphosphate (ppGpp or pppGpp, represented as (p)ppGpp), cyclic adenosine 3',5'-monophosphate (cAMP), cyclic dimeric GMP (c-di-GMP), cyclic guanosine 3',5'-monophosphate (cGMP), and cyclic dimeric AMP (c-di-AMP), and the gaseous nitric oxide (NO). The discussion focuses on processes central to cyanobacteria, such as nitrogen fixation, light perception, photosynthesis-related processes, and gliding motility. In addition, we address future research trajectories needed to better understand the signaling networks and cross talk in the signaling pathways of these molecules in cyanobacteria. Second messengers have significant potential to be adapted as technological tools and we highlight possible novel and practical applications based on our understanding of these molecules and the signaling networks that they control.

ppGpp in Sinorhizobium meliloti: biosynthesis in response to sudden nutritional downshifts and modulation of the transcriptome

Sinorhizobium meliloti Rm2011 responds to sudden shifts to nitrogen or carbon starvation conditions by an accumulation of the stringent response alarmone ppGpp and remodelling of the transcriptome. The gene product of relA, Rel(Sm) , responsible for synthesis of ppGpp, shows functional similarities to E. coli SpoT. Using promoter-egfp gene fusions, we showed that in Rm2011 relA is expressed at a low rate, as a readthrough from the rpoZ promoter and from its own weak promoter. The low level of relA expression is physiologically relevant, since overexpression of Rel(Sm) inhibits ppGpp accumulation. The N-terminal portion of Rel(Sm) is required for ppGpp degradation in nutrient-sufficient cells and might be involved in regulation of the ppGpp synthase and hydrolase activities of the protein. Expression profiling of S. meliloti subjected to sudden nitrogen or carbon downshifts revealed that repression of 'house-keeping' genes is largely dependent on relA whereas activation of gene targets of the stress sigma factor RpoE2 occurred independently of relA. The regulatory genes nifA, ntrB, aniA and sinR, as well as genes related to modulation of protein biosynthesis and nucleotide catabolism, were induced in a relA-dependent manner. dksA was required for the majority of the relA-dependent regulations.

Redox control of ntcA gene expression in Synechocystis sp. PCC 6803. Nitrogen availability and electron transport regulate the levels of the NtcA protein

In this work we have studied the influence of the cellular redox status in the expression of the Synechocystis sp. PCC 6803 ntcA gene. Two different ntcA transcripts with different 5' ends were detected, depending on the different dark/light or nitrogen availability conditions. Accumulation of a 0.8-kb ntcA message was light and nitrogen dependent, whereas a longer 1.2-kb ntcA transcript was neither light nor nitrogen regulated. NtcA protein levels increased concomitantly with the accumulation of the 0.8-kb ntcA transcript. The light-dependent accumulation of the ntcA gene and the NtcA protein was sensitive to electron transport inhibitors. In addition, Glc-grown Synechocystis sp. cells showed a similar ntcA expression pattern in darkness to that observed under illumination. These data suggested that electron transport, and not light per se may regulate ntcA gene expression. Primer extension analysis, together with gel mobility-shift assays, demonstrated that in vitro, the Synechocystis sp. NtcA protein specifically bound to the putative promoter region from the light/nitrogen-dependent ntcA transcript but not to that from the constitutive 1.2-kb ntcA mRNA. Band-shift experiments carried out in the presence of thiol oxidizing/modifiying agents and different reducing/oxidizing conditions suggested that NtcA binding to its own promoter was under a thiol-dependent redox mechanism. Our results suggest that the cellular redox status plays a central role in the autoregulatory mechanism of the NtcA protein.

Redox control of psbA gene expression in the cyanobacterium Synechocystis PCC 6803. Involvement of the cytochrome b(6)/f complex

We investigated the role of the redox state of the photosynthetic and respiratory electron transport chains on the regulation of psbA expression in Synechocystis PCC 6803. Different means to modify the redox state of the electron carriers were used: (a) dark to oxidize the whole electron transport chain; (b) a shift from dark to light to induce its reduction; (c) the chemical interruption of the electron flow at different points to change the redox state of specific electron carriers; and (d) the presence of glucose to maintain a high reducing power in darkness. We show that changes in the redox state of the intersystem electron transport chain induce modifications of psbA transcript production and psbA mRNA stability. Reduction of the intersystem electron carriers activates psbA transcription and destabilizes the mRNA, while their oxidation induces a decrease in transcription and a stabilization of the transcript. Furthermore, our data suggest that the redox state of one of the electron carriers between the plastoquinone pool and photosystem I influences not only the expression of the psbA gene, but also that of other two photosynthetic genes, psaE and cpcBA. As a working hypothesis, we propose that the occupancy of the Q(0) site in the cytochrome b(6)/f complex may be involved in this regulation.

Alterations in the accumulation of adenylylated nucleotides in heavy-metal-ion-stressed and heat-stressed Synechococcus sp. strain PCC 6301, a cyanobacterium, in light and dark

Heavy-metal-ion- (Cd2+, Cu2+, Pb2+, Hg2+ and Zn2+) or heat (50 degrees C)-stress treatments of the unicellular cyanobacterium Synechococcus sp., strain PCC 6301, under both light and dark conditions led to the accumulation of bis(5'-nucleosidyl)oligophosphates: Ap4A, Ap4G, Ap3A, Ap3G and Ap3Gp2. Under light regimens, the accumulation of Ap4A and Ap4G is more characteristic of heavy-metal-ion-stressed cells, whereas the accumulation of Ap3A, Ap3G and Ap3Gp2 is the dominant feature of heavy-metal-ion or heat-shock treatment during energy deprivation (i.e. in the dark). This accumulation of bisnucleoside oligophosphates supports a model whereby the adenylylated nucleotides are synthesized by the backward reaction of tRNA-aminoacyl synthetases. These nucleotides may also act to switch or modulate cyanobacterial responses under various environmental stress conditions.

AS-1 cyanophage infection inhibits the photosynthetic electron flow of photosystem II in Synechococcus sp. PCC 6301, a cyanobacterium

In Synechococcus sp. cells AS-1 cyanophage infection gradually inhibits the photosystem II mediated photosynthetic electron flow whereas the activity of photosystem I is apparently unaffected by the cyanophage infection. Transient fluorescence induction and flash-induced delayed luminescence decay studies revealed that the inhibition may occur at the level of the secondary acceptor, QB of photosystem II. In addition, the breakdown of D1-protein is inhibited, comparable to DCMU-induced protection of D1-protein turnover, in AS-1-infected cells.

Microbial stress proteins

There is general agreement that a function, perhaps the major function, of stress proteins under normal physiological conditions is to help assembly and disassembly of protein complexes and to catalyse protein-translocation processes. It remains unclear, however, as to what role these processes play in stressed cells. It could be that cells under stress produce abnormal, misfolded or otherwise damaged proteins and that increased synthesis of stress proteins is required to counter protein modifications. A role for stress proteins in recovery of cells from stress, as opposed to a role in helping cells to withstand a lethal stress, is thus suggested. The intracellular location of stress proteins, in the unstressed and stressed cell, is worthy of further studies. Members of the hsp70 family are associated with the cytosol, mitochondria and endoplasmic reticulum. There is evidence, particularly from studies on mammalian cells (Tanguay, 1985; Welch and Mizzen, 1988; Arrigo et al., 1988), that following stress hsps migrate to various cellular compartments and subsequently delocalize after stress. However, there is little comparable data from microbial systems for this phenomenon (e.g. Rossi and Lindquist, 1989). The question as to the role of stress proteins in the transient acquisition of thermotolerance remains to be answered. It is insufficient to equate the kinetics of stress-protein synthesis with acquisition of thermotolerance. Quantitative data on the amount of stress protein present at various times, including the recovery period, is required. The demonstration that microbial stress proteins are important antigenic determinants of micro-organisms causing major debilitating diseases in the world is an exciting observation. Studies on the interplay of pathogen and host, both carrying similar antigenic hsp determinants, will be a challenging area for future research. It is likely that E. coli and Sacch. cerevisiae, with their well-established biochemical and genetic properties, will continue to be the experimental systems of choice for studies on stress proteins. On the other hand, it is encouraging that studies on other micro-organisms have expanded in the past few years and have made substantial contributions towards our understanding of the stress response. The ubiquitous nature of the stress response and the remarkable evolutionary conservation of the stress proteins continue to be attractive areas for research.

Genetic relationship of two highly studied Synechococcus strains designated Anacystis nidulans

The cyanobacteria Synechococcus sp. strain PCC 7942 and Synechococcus sp. strain PCC 6301 are very closely related and both have been designated by the binomial Anacystis nidulans. The only established difference between the two strains is the superior transformation properties of strain PCC 7942. Significant homology between the rRNA genes of these strains was demonstrated by the ability of an rRNA operon from strain PCC 6301, interrupted by a spectinomycin and streptomycin resistance marker, to transform strain PCC 7942 by recombining with and replacing an endogenous rRNA operon. Restriction fragment length polymorphism data indicated that the chromosomes of the two strains were conserved around the three psbA loci, the two rRNA operons, and the psbDI locus. However, multiple polymorphisms were detected downstream of the psbDII locus, identifying a DNA rearrangement such as an inversion, insertion, or deletion within the chromosome. Analysis of genome structure by pulsed-field gel electrophoresis of large NotI restriction fragments showed only two bands that were visibly shifted between the chromosomes of the two strains. These data support their very close genetic relationship and the feasibility of studying genes derived from strain PCC 6301 in the highly transformable PCC 7942 strain.

Regulation of cyanobacterial pigment-protein composition and organization by environmental factors

The coordinate expression of stress-specific genes is a common response of all organisms to altered environmental conditions. In cyanobacteria, the physiological consequences of stress are often reflected in both the ultrastructure of the cell and in photosynthesis-related properties. This review will focus on the alterations in cyanobacterial pigment-protein organization which occur under different growth conditions, and how several molecular genetic aproaches are being used in this laboratory to investigate the regulatory mechanisms underlying these alterations. We will discuss in detail the response to iron starvation, and present a testable hypothesis for the mechanism of thylakoid reorganization mediated by this response.