| Popis: |
The changing environmental conditions in diverse habitats of cyanobacteria result in adaptive evolution involving both genetic and physiological modulation in order to cope up with extreme environments. One of the major common factors observed under different extreme conditions is denaturation of proteins, and accumulation of denatured proteins in the cells would result in lethality. Cells prevent accumulation of denatured proteins by synthesizing heat shock proteins, which function as chaperones and/or proteases. In cyanobacteria, the chaperonins or the Hsp60 family of proteins are the most predominant HSPs, unlike DnaK or the Hsp70 family in most other bacteria. Most cyanobacterial species have two groEL genes, one present as a part of a bicistronic groESL operon and the other as a monocistronic groEL2/cpn60 gene. Both the genes have been found to be essential for cyanobacterial growth, with groEL2 being dispensable in a few cyanobacterial species under ambient growth temperatures. The synthesis of the two proteins was induced in response to not only heat stress but several other abiotic stresses, suggesting a more global role for the chaperonins in cyanobacteria. Their expression was regulated mainly through the repressor protein HrcA which bound to the CIRCE element overlapping with the promoter region, with other regulatory elements, such as the K-box and H-box also playing a role during heat and/or light stresses. The two chaperonins exhibited differences in their biochemical activities, which possibly reflected on their distinct physiological roles. The cyanobacterial GroEL were distinct from most other bacterial GroEL in being able to function optimally in a GroES- and ATP-independent manner. Taken together, the multiple cyanobacterial Hsp60 proteins and their cis and trans regulation upon varying physiological conditions provide an in-depth insight into how the ancient chaperonins might have evolved, functioned, and contributed toward adaptive evolution and diversification of the more modern bacteria. |
