Stress responses of Synechocystis sp strain PCC 6803 mutants impaired in genes encoding putative alternative sigma factors

In the complete genome sequence of the cyanobacterium Synechocystis sp. str ain PCC 6803 [Kaneko et al. (1996). DNA Res 3, 109-136] genes were identifi ed encoding putative group 3 sigma -factors SigH (SII-0856), SigG (Sir-1545 ) and SigF (Sir-1564) and the regulatory protein RsbU (Sir-2031). Mutations in these genes were generated by interposon mutagenesis to study their imp ortance in stress acclimation. For the genes sigH, sigF and rsbU, the loci segregated completely. However, attempts to mutagenize the sigG locus resul ted in merodiploids. Under standard growth conditions only minor difference s were detected between the mutants and wild-type. However, cells of the Rs bU mutant showed a clear defect in regenerating growth after a nitrogen- an d sulphur-starvation-induced stationary phase. After applying salt, heat an d high-light shocks, stress protein synthesis was analysed by means of one- and two-dimensional electrophoresis. Cells of the SigF mutant showed a sev ere defect in the induction of salt stress proteins. Although the acclimati on to moderate salt stress up to 684 mM NaCl was not significantly changed in this mutant, its ability to acclimate to higher concentrations of NaCl w as reduced. Northern blot experiments showed a constitutive expression of t he rsbU and sigF genes. The expression of the sigH gene was found to be str ess-stimulated, particularly in heat-shocked cells, whilst that of sigG was transiently decreased under stress conditions. Possible functions of these regulatory proteins in stress acclimation of Synechocystis cells are discu ssed.