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Expression of the Secondary Sigma Factor ^X in Streptococcus pyogenes Is Restricted at Two Levels

Department of Microbiology and Immunology, Emory University School of Medicine, Atlanta, Georgia 30322

Received 25 March 2003/ Accepted 9 May 2003

Secondary RNA polymerase sigma factors in many bacteria are responsible for regulating a vast range of processes including virulence. A protein (^X) in the gram-positive human pathogen Streptococcus pyogenes (the group A Streptococcus or GAS) was recently shown to function in vitro as a secondary sigma factor. We report here the isolation of a mutant in which both sigX genes are inactivated, show that ^X functions in GAS cells, and show that the amount of ^X is controlled at two levels. Primer extension analysis indicates that sigX transcription is low in GAS cells grown in Todd-Hewitt yeast broth, and immunoblot assays with a ^X-specific polyclonal antibody demonstrate that the protein does not accumulate in these cells. To increase the level of sigX transcription in GAS, we constructed a strain that constitutively expresses the sigX gene from a heterologous promoter. Expression of sigX from this promoter led to transcription of the ^X-dependent cinA promoter in GAS cells. We found that expression of the sigX gene in a clpP mutant strain resulted in greater accumulation of ^X protein, which resulted in higher levels of transcription from the ^X-dependent promoters cinA, smf, and cglA. In addition, a clpP mutant containing sigX only at its wild-type loci on the chromosome generated more transcription from the ^X-dependent cinA promoter than did the wild-type parental strain. Therefore, ^X activity in GAS is limited by low-level transcription of the sigX structural genes and by clpP, which appears to negatively regulate ^X accumulation.

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