Properties of CsnR, the Transcriptional Repressor of the Chitosanase Gene, csnA, of Streptomyces lividans

A palindromic sequence is present in the intergenic region preceding the chitosanase gene csnA (SSPG_06922) of Streptomyces lividans TK24. This sequence was also found in front of putative chitosanase genes in several other actinomycete genomes and upstream genes encoding putative transcriptional regulators of the ROK family, including csnR (SSPG_04872) in S. lividans. The latter was examined as a possible transcriptional regulator (CsnR) of chitosanase gene expression. In vitro, purified CsnR bound strongly to the palindromic sequences of the csnA and csnR genes (equilibrium dissociation constant [K-D] = 0.032 and 0.040 nM, respectively). Binding was impaired in the presence of chitosan oligosaccharides and D-glucosamine, and chitosan dimer was found to be the best effector, as determined by an equilibrium competition experiment and 50% inhibitory concentration (IC50) determination, while glucose, N-acetyl-glucosamine, and galactosamine had no effect. In vivo, comparison of the S. lividans wild type and Delta CsnR strains using beta-lactamase reporter genes showed that CsnR represses the expression of csnA and of its own gene, which was confirmed by quantitative PCR (qPCR). CsnR is localized at the beginning of a gene cluster, possibly an operon, the organization of which is conserved through many actinomycete genomes. The CsnR-mediated chitosanase regulation mechanism seems to be widespread among actinomycetes.