The region of the IncW plasmid R388 involved in conjugal DNA metabolism and mobilization (MOBw) has been analyzed by Tn5tac1 insertion mutagenesis, genetic complementation and DNA sequencing. Three genes (trwA, trwB and trwC) were mapped within MOBw. They are transcribed from the same strand and away from oriT. The predicted products of trwA, trwB and trwC are proteins of 121, 507 and 966 amino acids, respectively. The three proteins were visualized in a minicell expression system, showing apparent molecular masses of 13.5, 55 and 105 kDa, respectively. The deduced amino acid sequence of TrwA shows significant similarity to TraJ of the IncP plasmids RP4 and R751, to NikA of the IncI plasmid R64 and to MobB of plasmid pTF-FC2. The amino acid sequence of TrwB predicts an integral membrane protein which contains an NTP-binding motif. It shows 28% to 29% identity with TraD of plasmids F and R100, 23% identity with TraG of plasmids RP4 and R751 and 20% identity with VirD4 of the Ti plasmids of Agrobacterium tumefaciens. The amino acid sequence of TrwC shows the characteristic motifs of the Rep family of DNA helicases. It shows 33% identity with the sequence of helicase I (TraI) of plasmid F. The similarity is highest in the N-terminal segments of the proteins, which show conservation of characteristic amino acid motifs of a family of DNA-relaxases, including VirD2 of the Ti plasmid. The conserved features of these three proteins among the different transfer systems suggest that a very widespread conjugal DNA mobilization mechanism is shared by the transfer apparatuses of IncF, IncI, IncP, IncW and Ti plasmids.