A secondary RNA polymerase sigma factor from Streptococcus pyogenes

The important human pathogen Streptococcus pyogenes (the group A streptococ cus or GAS) causes diseases ranging from mild, self-limiting pharyngitis to severe invasive infections. Regulation of the expression of GAS genes in r esponse to specific environmental differences within the host is probably k ey in determining the course of the infectious process, however, little is known of global regulators of gene expression in GAS. Although secondary RN A polymerase sigma factors act as global regulators of gene expression in m any other bacteria, none has yet been isolated from the GAS. The newly avai lable GAS genome sequence indicates that the only candidate secondary sigma factor is encoded by two identical open reading frames (ORFS). These ORFS encode a protein that is 40% identical to the transcription factor Comx, be lieved to act as an RNA polymerase sigma factor in Streptococcus pneumoniae . To test whether the GAS ComX homologue functions as a sigma factor, we cl oned and purified it from Escherichia coli. We found that in vitro, this GA S protein, which we call sigma (X), directed core RNA polymerase from Bacil lus subtilis to transcribe from two GAS promoters that contain the cin-box region, required for transcription by S. pneumoniae ComX in vivo. On the ot her hand, GAS sigma (X) did not promote transcription of a GAS promoter (ha sA) expected to be dependent on sigma (A), the housekeeping or primary RNA polymerase sigma factor. Addition of monoclonal antibody that inhibited sig ma (A)-directed transcription had no effect on sigma (X)-directed transcrip tion, showing that the latter was not the result of contaminating sigma (A) . Transcription of both cin-box-containing promoters initiated downstream o f the cin-box and two different single basepair substitutions in the cin-bo x of the cinA promoter each caused a severe reduction of sigma (X)-directed transcription in vitro. Thus, the cin-box is required for sigma (X)-direct ed transcription.