Differential expression of genes in the vir regulon of Streptococcus pyogenes is controlled by transcription termination.

Streptococcal C5a peptidase (SCP), encoded by scpA in Streptococcus pyogenes, is a surface molecule which is able to cleave and inactivate the chemotactic factor C5a. The scpA gene is part of the vir regulon and subject to positive regulation by the Mga protein. It is down-regulated compared to another Mga-activated gene, emm. A chloramphenicol acetyltransferase (CAT) reporter gene was used to measure scpA promoter activity. Previous work had shown that when a large portion of the scpA promoter region was deleted, expression of CAT increased relative to the wild-type. This deleted region was found to contain an inverted repeat. In this study we show that the inverted repeat in the leader mRNA is the site of transcription termination, which down-regulates expression of scpA. This is a novel mechanism for regulation of gene expression in S. pyogenes. A specific deletion of the inverted repeat in the scpA promoter-CAT reporter construct was made using inverse PCR. Expression was measured from single-copy chromosomal integrants. When the inverted repeat was deleted, expression increased. Furthermore, Northern hybridization confirmed the existence of a truncated transcript, consistent with a transcription termination mechanism.