To determine if the 89K PAI harbours a functional GI-type T4SS, mobilization experiments were performed to examine if the transferability of 89K decreased by making individual knockouts of the T4SS gene homologues. Table 1 shows the results of conjugation experiments between these T4SS gene homologue mutants and the recipient strain SS2-NDrecA. No transconjugants were obtained when DmobA89K, DmobC89K, DvirB4-89K, DvirB6-89K and DvirD4-89K were used as donors. However, a DvirB1-89K derivative of 89K was still able to transfer at a frequency of 9.2 ¥ 10 -7 , approximately sixfold less than that of 89K*. A similar phenomenon had been reported that the transfer capacity of the IncQ plasmid RSF1010 was reduced approximately 10-fold in a virB1 mutant (Bohne et al., 1998) . All of these transfer deficiencies could be fully or partially restored by transcomplementation (Table 1) . Thus, these results indicated that the GI-type T4SS genes are absolutely required for 89K transfer, with the exception of virB1-89K (which contributes to but is not essential for 89K transfer).