A new staphylococcal sigma factor in the conserved gene cassette: functional significance and implication for the evolutionary processes

Background: Staphylococcus aureus is a major human pathogen and causes a serious hospital infection due to the acquired multidrug resistance. Unlike the well-studied bacteria such as Escherichia coli and Bacillus subtilis , which have seven and 18 sigma factors, respectively, only two sigma factors have been known for S. aureus . We searched for possible sigma factor genes by examining the S. aureus genome with a special attention to the gene arrangement around the sigma factor genes of a close relative, B. subtilis . Results: A new sigma factor gene was identified in Staphylococcus . The gene constituted a conserved gene cluster with other genes including translation- and transcription-related genes. Phylogenetic analysis and comparison of the gene sequences among species indicated that the staphylococcal sigma factor originated from a common ancestor of B. subtilis SigH. An over-expression of this sigma factor in S. aureus resulted in a drastic induction of the expression of the com operons that encode proteins required for the natural genetic competence. Conclusions: We demonstrated that the newly identified staphylococcal sigma factor participated in a regulatory network of transcription that controlled the genetic competence genes. In our phylogenetic tree, the factor was classified as a single group with a common function.