Monday, 7 April 2014

Heat-shock response of cyanobacteria

Cyanobacteria originated as a group of photoautotrophs nearly 3.5 billion years ago and were partially responsible for the initial oxygenation of the earth’s atmosphere. Today they are ubiquitous across aquatic ecosystems, even in extreme environments such as, hot springs, frozen lakes, salt ponds and deserts. They have the ability to survive in temperatures ranging from -60 to 74˚C. Studies on the stress responses of cyanobacteria are essential, as they play a major role in ecosystems as well as being used for human purposes in biotechnology and as biofertilizers.

Cyanobacteria are nitrogen fixers in which this process depends entirely on photosynthesis and both these processes are affected by heat and other stressors. Photosystems can become inactive as a result of a temperature shift, which has been shown in some unicellular cyanobacteria. Nitrogen fixation has also been found to be fragile at temperatures above 42˚C. These changes in metabolic capabilities in the presence of environmental stressors, emphasizes the importance of studying the heat shock response in cyanobacteria.

Cyanobacteria induce a group of proteins called heat-shock proteins (Hsps) by transcriptional activation and the degree of temperature increase determines the magnitude in which genes are upregulated. A typical heat-shock response in Synechocystis PCC6803 includes the upregulation of 90 genes after one hour of heat stress. The protein family Hsp100 are enhanced mainly by heat stress, whereas the Hsp90 family is associated with several abiotic stressors, a more general stress protein.

Molecular chaperones are essential components of many cellular functions: 1) folding of proteins after translation 2) transport of unfolded proteins 3) protein conformational homeostasis and 4) protection of the photosynthetic apparatus from stress induced damage. Hsps are present in low abundances under normal conditions, but are amplified when needed. These genes are then repressed by cis-elements or by trans-acting proteins. Maintenance of chaperone proteins in appropriate levels ensures there availability on demand and reduces any complications that might arise.

In conclusion, the roles of Hsp in cyanobacteria may help to enhance their stress tolerance, especially when subjected to sub-lethal levels prior to exposure. This was a fairly complex paper and went into a lot of detail about complicated pathways involving genes. The language used was also difficult to understand and maybe aimed mainly for geneticists.

Rajaram, H., Chaurasia, A.K. and Apte, S.K. (2014) Cyanobacterial heat-shock response: role and regulation of molecular chaperones. Microbiology. 160: 647-658

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