Figure 1.
Diagram of the regulatory scheme for flagellar and toxin phase variation in C. difficile. A DNA invertible element, which we termed the “flagellar switch,” is located in the 5′ UTR of the flgB operon and controls transcription of structural and regulatory genes necessary for flagellum biosynthesis and motility. One orientation of the flagellar switch, but not the other, is permissive for downstream gene expression. In addition, the flagellar switch regulates toxin production by controlling the expression of sigD, which located in the flgB operon and encodes a sigma factor, σD, that promotes toxin gene transcription. RecV, a tyrosine recombinase, catalyzes DNA inversion in both directions at the flagellar and cwpV switches, and also impacts one or more unidentified genetic switches that affect colony morphology.