Caulobacter crescentus pili: structure and stage-specific expression.

Pili are functionally expressed during the predivisional and swarmer stages of the Caulobacter crescentus differentiation cycle. They appear on the developing swarmer pole and at the same cellular location as flagella and the phiCbK receptor sites. Pili disappear when the swarmer cell differentiates into a stalked cell; this occurs with the loss of flagella and the disappearance of phage receptor sites. C. crescentus CB13B1a pili have been purified and characterized. Monomeric pilin is a protein with an apparent molecular weight of 8,500 that stains weakly with periodic acid-Schiff reagent. The amino acid composition of purified pilin reveals very low quantities of basic amino acids and a complete absence of methionine. Pilin is synthesized throughout the C. crescentus differentiation cycle. Neither free pili nor pilin monomers are detectable in the growth media, suggesting that loss of piliation in the swarmer- to stalked-cell transition occurs via pilus retraction.