Rhodobacter sphaeroides LexA has dual activity: optimising and repressing recA gene transcription

摘要

Transcription of the Rhodobacter sphaeroides recA promoter (P_(recA)) is induced upon DNA damage in a lexA-dependent manner. In vivo experiments demonstrate that lexA protein represses and might also activate transcription of P_(recA) region. In vitro transcription analysis, using Escherichia coli RNA polymerase (RNAP), indicated that the presence of LexA may stimulate and repress transcription of P_(recA). EMSA and DNase I footprinting experiments show that LexA and RNAP can bind simultaneously to P_(recA). At low LexA concentrations it enhances RNAP binding to P_(recA), stimulates open complex formation and strand separation beyond the transcription start site. At high LexA concentrations, however, RNAP-promoted strand separation is not observed beyond the +5 region. LexA might repress transcription by interfering with the clearance process instead of blocking the access of RNAP to the promoter region. Based on these findings we propose that the R.sphaeroides LexA protein performs find tuning of the SOS response, which might provide a physiological advantage by enhancing transcription of SOS genes and delaying full activation of the response.