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.