T4SS-mediated transfer of 89K PAI in S. suis 2 1679 complex. However, it should be noted that some other factor(s) (e.g. muramidase) may also be implicated in the formation of the transmembrane pore because inactivation of virB1-89K did not completely abolish the conjugal transfer capacity of 89K. VirB6-89K is a highly hydrophobic protein containing six predicted transmembrane segments and is the best candidate for a channel-forming protein in A. tumefaciens (Christie, 1997) . Adjacent genes encoding peptides (e.g. 05SSU0970, 05SSU0972 and 05SSU0975) harbouring transmembrane domains may also contribute to the construction of the transport apparatus. After the conjugation machinery is manufactured, the VirB4-89K ATPase would work as an alternative translocation energy supplier, aside from VirD4-89K, to motor the DNA substrate through the mating channel and drive export as in A. tumefaciens (Dang et al., 1999) .