FIG 9.

Schematic model for DNA uptake in T. thermophilus. In stage I, the pilin PilA4 (17) is assembled into a competence pilus fiber by the inner membrane complex PilMNO and PilC (14, 73). Elongation of the pilus is powered by the hexameric AAA-ATPase PilF, which provides energy for the assembly via ATP hydrolysis (13, 74). In stage II, double-stranded DNA is bound by the pilus tip-associated DNA receptor ComZ/PilA2 and transported into the periplasm through the secretin pore PilQ (24). A competence-associated retraction ATPase has not been identified and may not, in any case, be required. Once in the periplasm, double-stranded DNA binds to ComEA (6), one strand is degraded by an unidentified endonuclease, and the remaining DNA strand is translocated across the inner membrane through ComEC (20). In the cytoplasm, DNA is used as a source for intracellular metabolism or recombined with the host chromosomal genome (75).