Based on our experimental results presented here, in combination with previous studies on the Agrobacterium oncogenic T-DNA transfer system (Christie and Vogel, 2000; Atmakuri et al., 2004; Middleton et al., 2005; Guo et al., 2007) , we would like to propose a model for the transfer mechanism of the 89K PAI (Fig. 6) . In a small proportion of host bacteria, the 89K-borne integrase mediates precise excision of 89K from the chromosome with the help of an excisionase. Upon detection of a mating signal, the transfer of the excised circular intermediate of 89K begins (Llosa et al., 2002) . In the first step, MobA89K initiates the processing reaction by generating a strandand site-specific nick at the oriT site of 89K, with help from the auxiliary MobC89K protein. Next, the 05SSU0934 DNA helicase unwinds the nicked strand during relaxosome formation, and the VirD4-89K coupling protein then links the relaxosome and leads it directly to the mating channel (Llosa et al., 2003) . The VirB1-89K lytic transglycosylase, for which peptidoglycan cleavage activity has been previously demonstrated in A. tumefaciens (Zupan et al., 2007) , should play a crucial role in local opening of the peptidoglycan during the assembly of the transenvelope transporter The 89K-borne integrase mediates precise excision of 89K from the chromosome with the aid of excisionase. After receiving a mating signal, the transfer initiates with the oriT nicking reaction catalysed by the MobA89K and MobC89K proteins. The cleaved strand is then unwound by the helicase and delivered to the 89K-encoded GI-type T4SS machinery. The VirB1-89K component is thought to act as a lytic transglycosylase, locally opening the peptidoglycan with contributions from some other factor(s) during the assembly of the transport channel. VirD4-89K is a coupling protein required to deliver the DNA substrate to the transport channel, and together with VirB4-89K, required to energize the substrate transport across the channel. The highly hydrophobic VirB6-89K homologue constitutes the main portion of the transport channel.