Cloning and characterization of the major groESL operon from a nitrogen-fixing cyanobacterium Anabaena sp. strain L-31

GroEL of the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31 exhibits GroES and ATP-independent refolding activity

The nitrogen-fixing cyanobacterium, Anabaena L-31 has two Hsp60 proteins, 59 kDa GroEL coded by the second gene of groESL operon and 61 kDa Cpn60 coded by cpn60 gene. Anabaena GroEL formed stable higher oligomer (>12-mer) in the presence of K(+) and prevented thermal aggregation of malate dehydrogenase (MDH). Using three protein substrates (MDH, All1541 and green fluorescent protein), it was found that the refolding activity of Anabaena GroEL was lower than that of Escherichia coli GroEL, but independent of both GroES and ATP. This correlated with in vivo data. GroEL exhibited ATPase activity which was enhanced in the presence of GroES and absence of a denatured protein, contrary to that observed for bacterial GroEL. However, a significant role for ATP could not be ascertained during in vitro folding assays. The monomeric Cpn60 exhibited much lower refolding activity than GroEL, unaffected by GroES and ATP. In vitro studies revealed inhibition of the refolding activity of Anabaena GroEL by Cpn60, which could be due to their different oligomeric status. The role of GroES and ATP may have been added during the course of evolution from the ancient cyanobacteria to modern day bacteria enhancing the refolding ability and ensuring wider scope of substrates for GroEL.

Cyanobacterial heat-shock response: role and regulation of molecular chaperones

Cyanobacteria constitute a morphologically diverse group of oxygenic photoautotrophic microbes which range from unicellular to multicellular, and non-nitrogen-fixing to nitrogen-fixing types. Sustained long-term exposure to changing environmental conditions, during their three billion years of evolution, has presumably led to their adaptation to diverse ecological niches. The ability to maintain protein conformational homeostasis (folding–misfolding–refolding or aggregation–degradation) by molecular chaperones holds the key to the stress adaptability of cyanobacteria. Although cyanobacteria possess several genes encoding DnaK and DnaJ family proteins, these are not the most abundant heat-shock proteins (Hsps), as is the case in other bacteria. Instead, the Hsp60 family of proteins, comprising two phylogenetically conserved proteins, and small Hsps are more abundant during heat stress. The contribution of the Hsp100 (ClpB) family of proteins and of small Hsps in the unicellular cyanobacteria (Synechocystis and Synechococcus) as well as that of Hsp60 proteins in the filamentous cyanobacteria (Anabaena) to thermotolerance has been elucidated. The regulation of chaperone genes by several cis-elements and trans-acting factors has also been well documented. Recent studies have demonstrated novel transcriptional and translational (mRNA secondary structure) regulatory mechanisms in unicellular cyanobacteria. This article provides an insight into the heat-shock response: its organization, and ecophysiological regulation and role of molecular chaperones, in unicellular and filamentous nitrogen-fixing cyanobacterial strains.

Overexpression of the groESL operon enhances the heat and salinity stress tolerance of the nitrogen-fixing cyanobacterium Anabaena sp. strain PCC7120

The bicistronic groESL operon, encoding the Hsp60 and Hsp10 chaperonins, was cloned into an integrative expression vector, pFPN, and incorporated at an innocuous site in the Anabaena sp. strain PCC7120 genome. In the recombinant Anabaena strain, the additional groESL operon was expressed from a strong cyanobacterial P(psbA1) promoter without hampering the stress-responsive expression of the native groESL operon. The net expression of the two groESL operons promoted better growth, supported the vital activities of nitrogen fixation and photosynthesis at ambient conditions, and enhanced the tolerance of the recombinant Anabaena strain to heat and salinity stresses.

Nitrogen status and heat-stress-dependent differential expression of the cpn60 chaperonin gene influences thermotolerance in the cyanobacterium Anabaena

Heat stress caused rapid and severe inhibition of photosynthesis and nitrate reduction in nitrate-supplemented cultures of the cyanobacterium Anabaena sp. strain L-31, compared to nitrogen-fixing cultures. Anabaena strains harbour two hsp60 family genes, groEL and cpn60, respectively encoding the 59 kDa GroEL and 61 kDa Cpn60 chaperonin proteins. Of these two Hsp60 chaperonins, GroEL was strongly induced during heat stress, irrespective of the nitrogen status of the cultures, but Cpn60 was rapidly repressed and degraded in heat-stressed nitrate or ammonium-supplemented cultures. The recovery of photosynthesis, nitrate assimilation and growth in heat-stressed, nitrate-supplemented cultures were preceded by resynthesis and restoration of cellular Cpn60 levels. Glutamine synthetase activity, although adversely affected by prolonged heat stress, was not dependent on either the nitrogen status or Cpn60 levels during heat stress. Overexpression of the Cpn60 protein in the closely related Anabaena sp. strain PCC7120 conferred significant protection from heat stress to growth, photosynthesis and nitrate reduction in the recombinant strain. The data favour a role for Cpn60 in carbon and nitrogen assimilation in Anabaena.

Effects of high light on transcripts of stress-associated genes for the cyanobacteria Synechocystis sp. PCC 6803 and Prochlorococcus MED4 and MIT9313

Cyanobacteria constitute an ancient, diverse and ecologically important bacterial group. The responses of these organisms to light and nutrient conditions are finely controlled, enabling the cells to survive a range of environmental conditions. In particular, it is important to understand how cyanobacteria acclimate to the absorption of excess excitation energy and how stress-associated transcripts accumulate following transfer of cells from low- to high-intensity light. In this study, quantitative RT-PCR was used to monitor changes in levels of transcripts encoding chaperones and stress-associated proteases in three cyanobacterial strains that inhabit different ecological niches: the freshwater strain Synechocystis sp. PCC 6803, the marine high-light-adapted strain Prochlorococcus MED4 and the marine low-light-adapted strain Prochlorococcus MIT9313. Levels of transcripts encoding stress-associated proteins were very sensitive to changes in light intensity in all of these organisms, although there were significant differences in the degree and kinetics of transcript accumulation. A specific set of genes that seemed to be associated with high-light adaptation (groEL/groES, dnaK2, dnaJ3, clpB1 and clpP1) could be targeted for more detailed studies in the future. Furthermore, the strongest responses were observed in Prochlorococcus MED4, a strain characteristic of the open ocean surface layer, where hsp genes could play a critical role in cell survival.

Heat-shock response and its contribution to thermotolerance of the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31

Compared to Escherichia coli, the nitrogen-fixing soil cyanobacterium Anabaena sp. strain L-31 exhibited significantly superior abilities to survive prolonged and continuous heat stress and recover therefrom. Temperature upshift induced the synthesis of heat-shock proteins of similar molecular mass in the two microbes. However, in Anabaena sp. strain L-31 the heat-shock proteins (particularly the GroEL proteins) were synthesised throughout the stress period, were much more stable and accumulated during heat stress. In contrast, in E. coli the heat-shock proteins were transiently synthesised, quickly turned over and did not accumulate. Nitrogenase activity of Anabaena cells of sp. strain L-31 continuously exposed to heat stress for 7 days rapidly recovered from thermal injury, although growth recovery was delayed. Exposure of E. coli cells to >4.5 h of heat stress resulted in a complete loss of viability and the ability to recover. Marked differences in the synthesis, stability and accumulation of heat-shock proteins appear to distinguish these bacteria in their thermotolerance and recovery from heat stress.

Cloning and characterization of the groE heat-shock operon of the marine bacterium Vibrio harveyi

The DNA region of the Vibrio harveyi chromosome containing the heat-shock genes groES and groEL was cloned, and the genes were sequenced. These genes are arranged in the chromosome in the order groES-groEL. Northern hybridization experiments with RNA from V. harveyi and a DNA probe carrying both groES and groEL genes showed a single, heat-inducible transcript of approximately 2200 nt, indicating that these genes form an operon. Primer extension analysis revealed a strong, heat-inducible transcription start site 59 nt upstream of groES, preceded by a sequence typical for the Escherichia coli heat-shock promoters recognized by the sigma(32) factor, and a weak transcription start site 25 nt upstream the groES gene, preceded by a sequence typical for sigma(70) promoters. Transcription from the latter promoter occurred only at low temperatures. The V. harveyi groE operon cloned in a plasmid in E. coli cells was transcribed in a sigma(32)-dependent manner; the transcript size and the sigma(32)-dependent transcription start site were as in V. harveyi cells. Comparison of V. harveyi groE transcription regulation with the other well-characterized groE operons of the gamma subdivision of proteobacteria (those of E. coli and Pseudomonas aeruginosa) indicates a high conservation of the transcriptional regulatory elements among these bacteria, with two promoters, sigma(32) and sigma(70), involved in the regulation. The ability of the cloned groESL genes to complement E. coli groE mutants was tested: V. harveyi groES restored a thermoresistant phenotype to groES bacteria and enabled lambda phage to grow in the mutant cells. V. harveyi groEL did not abolish thermosensitivity of groEL bacteria but it complemented the groEL mutant with respect to growth of lambda phage. The results suggest that the GroEL chaperone may be more species-specific than the GroES co-chaperone.