Constitutive expression of a small heat-shock protein confers cellular thermotolerance and thermal protection to the photosynthetic apparatus in cyanobacteria

The role of a small heat-shock protein (Hsp) in the acquisition of thermoto lerance in cyanobacteria was investigated. Synechococcus sp. PCC 7942 was t ransformed with an expression vector carrying the coding sequence of the hs pA gene encoding a small heat-shock protein from Synechococcus vulcanus und er the control of the tac promoter. The transformant which was shown to con stitutively express HspA displayed improved viability compared with the ref erence strain upon transfer from 30 to 50 degreesC in the light. When the h eat shock was given in darkness, the survival rate in the reference strain increased greatly, approaching: a level similar to that for the HspA expres sing strain after heat shock in the light. Expression of HspA increased the rmal resistance of photosystem ii (PS II) and protected phycocyanin from he at-induced photobleaching. Our results are indicative of a central role for HspA in amelioration of the harmful effect of light during heat stress and identified the possible sites of action of the small Hsp in vivo to be the PS II complex and the light-harvesting phycobilisomes. (C) 2000 Federation of European Biochemical Societies. Published by Elsevier Science B.V. All rights reserved.