Serotype conversion gene rfbT is directly regulated by histone-like nucleoid structuring protein (H-NS) in V....

Vibrio cholerae serogroup O1 (V. cholerae O1) is closely associated with cholera epidemics and has two main immunologically distinguishable serotypes, Ogawa and Inaba. Isolates serotype as Ogawa if the O-antigen polysaccharide (O-PS) is methylated or as Inaba if the O-PS is not methylated. This methylation is mediated by a methyltransferase encoded by the rfbT gene, and the mutation and low expression of rfbT results in serotype switch from Ogawa to Inaba. Previously, we have shown that cAMP receptor protein (CRP) activates rfbT. In this study, we demonstrated that histone-like nucleoid structuring protein (H-NS) is directly involved in the transcriptional repression of rfbT. This finding is supported by the analyses of rfbT mRNA level, rfbT-lux reporter fusions, electrophoretic mobility shift assay (EMSA), and DNase I footprinting assay. The rfbT mRNA abundances were significantly increased by deleting hns rather than fis which also preferentially associates with AT-rich sequences. A single-copy chromosomal complement of hns partly restored the down-regulation of rfbT. Analysis of rfbT-lux reporter fusions validated the transcriptional repression of hns. Subsequent EMSA and DNase I footprinting assay confirmed the direct binding of H-NS to rfbT promoter and mapped the exact binding site which was further verified by site-directed mutagenesis and promoter functional analysis. Furthermore, we found that in hns deletion mutant, CRP is no longer required for transcriptionally activating rfbT, suggesting that CRP functions as a dedicated transcription factor to relieve H-NS repression at rfbT. Together, this study expanded our understanding of the genetic regulatory mechanism of serotype conversion by global regulators in V. cholerae O1.