[Genetic analysis of sacR, a cis-regulator of levan-saccharase synthesis of Bacillus subtilis].

The phenotypical and physical characterization of point mutations and deletions affecting sacR were analysed. The 2 X 28-bp palindromic region present within sacR was responsible for sacB inducibility by sucrose. The sacR mutations also affected sacB expression in Escherichia coli, suggesting that the palindromic region is a transcriptional terminator. It is difficult to apply classical bacterial models of negative or positive regulation to sacR function in B. subtilis. We propose that sacR works as a transcriptional attenuator. Our hypothesis is similar to that proposed for the regulation of the tryptophan operon in B. subtilis: the attenuation of sacB transcription could be modulated by a diffusible regulator whose activity is controlled by the inducer, sucrose. Furthermore, we have observed very strong sequence homologies between sacR and the region located upstream from the gene of another secreted enzyme from B. subtilis, beta-glucanase. We propose a preliminary discussion of this observation.