MOBILIZATION OF CLOSELY RELATED PLASMIDS PUB110 AND PBC16 BY BACILLUS PLASMID PXO503 REQUIRES TRANS-ACTING OPEN READING FRAME-BETA

Genetic analysis of the closely related nonconjugative plasmids pUB110 and pBC16 has demonstrated that in open reading frame β (ORF-β) region in pUB110 and the corresponding homologous region in pBC16 are essential for mobilization of these plasmids by pLS20 or its derivatives. Deletions in this region or insertions that interrupted ORF-β severely impaired or eliminated the mobilization of pUB110::pUC18 and pBC16::pUC18 hybirds. In contrast, a hybrid in which pUC18 was inserted into pBC16 at a point outside ORF-β transferred at a frequency comparable to that of intact pUB110 or pBC16 (10-4 transcipients per donor cell). The defect of most transfer-deficient (Mob-) hybrid plasmids could be complemented by an intact sister plasmid (i.e., pBC16 for pUB110::pUC18 Mob- hybrids). The inability to complement certain constructs suggested that the origin of transfer might be located in an area 5' to ORF-β. Furthermore, cloning the region 5' to ORF-β onto a nonmobilizable pC194::pUC18 construct resulted in a hybrid plasmid, pUCCoriTBC16, that could be mobilized with complementation. These results indicate that mobilization of pUB110 and pBC16 by conjugative helper plasmids requires ORF-β in trans and at least one other region, including the RS(A) sequence, which presumably functions as an origin of transfer, in cis.