In Cis and In Trans Autorepression of the prqR Gene in Cyanobacterium Synechocystis sp. PCC 6803

Genetic analysis of the allele interactions was carried out with the use of recombinant plasmids and reporter genes to study the autorepressor function of prqR, which negatively regulates the prqR–prqA operon and the response to oxidative stress inducer methyl viologen (MV) in cyanobacterium Synechocystis sp. PCC 6803. The wild-type prqR cloned in Escherichia colishowed negative autoregulation and suppressed in trans the derepressed mutant alleles. Frameshift mutation C134fs, which was introduced in prqR by site-directed mutagenesis, impaired the autoregulation, implicating the PrqR C-terminal domain in transcriptional repression. Missense mutation C134S, changing the only redox-sensitive Cys of PrqR, had no effect on prqR expression, indicating that oxidation and consequent disulfide bridging of two PrqR molecules was not responsible for MV-induced autorepression of prqR. Analysis of the prqR–prqA deletion derivatives lacking the promoter and most of prqR revealed weak uncontrollable expression of reporter cat, testifying to the existence of a constitutive promoter in prqA responsible for MV resistance. The interaction of the wild-type and mutant prqR alleles in Synechocystis cells revealed a cis-dominant character of the impairment of prqR autoregulation. Stimulation of in cis autorepression of prqR was assumed to contribute to the induction of systems protecting cyanobacteria against oxidative stress.