Differential expression of genes in thevir regulon ofStreptococcus pyogenes is controlled by transcription termination

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Abstract
Streptococcal C5a peptidase (SCP), encoded byscpA inStreptococcus pyogenes, is a surface molecule which is able to cleave and inactivate the chemotactic factor C5a. ThescpA gene is part of thevir 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 measurescpA promoter activity. Previous work had shown that when a large portion of thescpA 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 ofscpA. This is a novel mechanism for regulation of gene expression inS. pyogenes. A specific deletion of the inverted repeat in thescpA 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.