Regulation of expression of heat-shock proteins in cyanobacteria and studies on multi-chaperone complexes

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1. Regulation of transcription of cyanobacterial heat-shock protein (Hsp) genes(1) A novel Hsp, Orf7.5, did not bind to 5'-upstream region of groEL operon from Synechococcus PCC 7942, but interacted with the major sigma factor, enabling it to bind to the region. We postulate that Orf7.5 together with the major sigma factor is involved in a positive regulation of the operon. (2) An hrcA disruptant of Synchocystis PCC 6803 was constructed. With the mutant, we showed that the groEL genes are regulated by a negative mechanism (CIRCE/HrcA system) and a positive one that is yet to be clarified. (3) We detected a protein in the unstressed cells of the thermophilic cyanobacteriurn Synechococcus that specifically binds to a 5'-untranslated region of the hspA gene that encodes a small Hsp homologue. The time course of the DNA binding by the protein and the hspA mRNA level were inversely correlated. (4) The heat shock response is generally characteirized by an immediate, intense and transient activation of gene expression. We found that light modulates these characteristics of the heat shock response in cyanobacteria.2. Post-transcriptional regulation of heat shock genes.A downstream box (DB) sequence was found to exist in the coding sequence of the hspA gene. We postulated that the interaction of DB with 16S rRNA results in the stabilization of the hspA mRNA and the increase of its translation.3. Cellular functions of HtpGPhenotypic analyzes of an htpG disruptant of Synechococcus PCC 7942 showed that HtpG plays roles in cold acclimation and protection from oxidative stress as well as in thermo-tolerance.4. Multi-chaperone complexA novel, high-molecular-weight complex containing GroEL, DnaK and an unknown 48-kDa protein accumulated at 16℃, but the accumulation was strongly inhibited in the htpG disruptant, indicating that HtpG is involved in the formation of the complex.