Contact between CrgA, a LysR-like regulatory protein in Neisseria meningitidis, and DNA is involved in the repression of several bacterial genes upon contact with epithelial cells. We used a defined in vitro system containing crgA promoter, purified RNA polymerase (RNAP) and purified CrgA protein to demonstrate that CrgA was directly responsible for this transcriptional repression. Interaction between the C-terminal domain of CrgA and the RNAP led to the production of short abortive transcripts, suggesting that CrgA may act by preventing RNAP from clearing the promoter. We probed the regulation by CrgA of its own production by analysing CrgA-DNA contacts during cell-bacteria interaction by assaying in vivo protection against dimethyl sulphate (DMS) methylation. Comparison of DMS footprints in vitro and in vivo suggested that CrgA repressed transcription through specific base contacts, probably in the major groove of the DNA double helix, resulting in DNA looping. Upon contact with target cells, CrgA was released from the DNA, allowing transcription of the target gene to proceed to elongation and facilitating tight control of the expression of genes regulated by CrgA.
Copyright 2004 Blackwell Publishing Ltd