Transcriptional, chemosensory and cell-contact-dependent regulation of type IV pilus expression

Expression of Type IV pili (Tfp), multifunctional surface appendages expressed by Gram-negative species of medical and environmental significance, has previously been shown to be regulated by consensus two-component systems. Elucidation of their unique biogenesis pathway and the dynamics of pilus growth and retraction involved in motility have revealed a novel step at which regulation might be imposed. Studies of Tfp expression following adherence to host tissue clearly demonstrate regulation by modulation of the retraction process. In addition, a large set of components related to flagellar chemosensory pathways has been shown to influence Tfp expression levels in many species. Like their flagellar counterparts, the Tfp-dedicated homologues are proposed to function by regulating motor function. Rather than dictating the switch frequencies of organelle rotation, however, they are hypothesized to control the rates of fiber extrusion and retraction.