Regulation of the Promoter for Capsular Polysaccharide Synthesis in Neisseria Meningitidis Serogroup B by HTH_XRE Family Transcription Factor

The capsular polysaccharide synthesis (cps) locus of Neisseria meningitidis is implicated in invasive meningococcal disease. The synthesis (synABCD) and transport (ctrABCD) operons are transcribed in opposite directions from a common intergenic region and expression is negatively regulated by the bacterial two-component system misR/misS and thermosensitive RNA folding. However, these mechanisms do not fully explain the stationary phase responses and the cis-acting elements remain to be fully characterized. Using GFP reporter gene and site-directed mutagenesis, cis-regulatory elements in the 134-bp intergenic region, NmIR, were investigated. While confirming a known RpoD promoter, an additional potential promoter element and putative binding sites for the transcription factors fis and lexA were identified through sequence analysis. Deletion of the putative LexA binding site led to an increase in GFP fluorescence. The N. meningitidis genome carries only one lexA homolog, the Helix-Turn-Helix regulator XRE family member (GenBank-NMB0910, HTH_XRE). Trans-complementation of the NmIR-GFP reporter with the N. meningitidis HTH_XRE expression plasmid led to increased fluorescence. Trans-complementation with either misR/misS or nusG decreased reporter gene expression. Consistent with previous reports, deletion of the RpoD promoter reduced expression by 50%, suggesting a redundancy of promoter elements in the intergenic region. Thus, the results confirm the functioning of an exogenous N. meningitidis CPS synthesis promoter in E. coli and demonstrate its regulation through trans-complementation by misR/misS, HTH_XRE, and nusG. ImportancePathogenic Neisseria meningitidis, a causal agent of bacterial meningitis, secretes capsular polysaccharides of different compositions that differentiate the serogroups. Since the capsule is an important virulence factor that determines adhesion to epithelia and ability to invade tissues, there is need to understand the underlying mechanisms for its expression. Furthermore, bacterial polysaccharides are potential sources of novel biomaterials. The expression of the capsule production genes is regulated, and this study reveals a mechanism involving a transcription factor, HTH_XRE, whose function in Neisseria meningitidis is not known. It extends the understanding of capsular expression regulation by identifying other control elements in the promoter region. The results will have applications in optimizing biomaterial production or in developing therapeutic interventions.