Heat shock proteins play a role in both high and low temperature stresses in cyanobacteria

Abstract

The role of heat shock proteins (Hsps) in the cyanobacterium Synechococcus PCC 7942 in the acquisition of high and low temperature tolerances has been investigated. (1) Role of small Hsp. Synechococcus PCC 7942 was transformed with an expression vector carrying the coding sequence of the hspA gene encoding a small Hsp from S. vulcanus under the control of the tac promoter. The transformant which constitutively expressed HspA displayed improved viability at a lethal temperature. Expression of HspA increased thermal resistance of photosystem II and the physical order of thylakoid membranes. It protected phycocyanin from heat-induced photobleaching. Approximately 40% of the total cellular HspA in 30oC-grown cell was localized in insoluble fractions of the cell extracts. Experiments such as immunogold localization of HspA using transmission electron microscopy are in progress in order to reveal the association of the small Hsp with thylakoids and phycobilisomes. (2) Role of HtpG. We inactivated the htpG gene by gene targeting. The mutant cells lost both basal and acquired thermotolerances, indicating that HtpG plays an essential role for the thermotolerance in cyanobacteria. Absence of HtpG greatly reduced thermal resistance of photosynthesis. The inactivation of the htpG gene also led to an inhibition of growth at 16oC. After the shift from 30oC to 16oC, the rate of photosynthesis in the wild type decreased initially and then recovered, while the rate in the mutant was inhibited but did not recover. These results indicate that HtpG plays a role in the acclimation to low temperatures. In order to compare the role of HtpG with that of small Hsp, the phenotype analysis of the htpG mutants expressing HspA consitutively is in progress.