Horizontal DNA transfer contributes significantly to the dissemination of antibiotic resistance genes in Bacteroides fragilis. To further our understanding of DNA transfer in B. fragilis, we isolated and characterized a new transfer factor, cLV25. cLV25 was isolated from B. fragilis LV25 by its capture on the nonmobilizable Escherichia coli-Bacteroides shuttle vector pGAT400⌬BglII. Similar to other Bacteroides sp. transfer factors, cLV25 was mobilized in E. coli by the conjugative plasmid R751. Using Tn1000 mutagenesis and deletion analysis of cLV25, two mobilization genes, bmgA and bmgB, were identified, whose predicted proteins have similarity to DNA relaxases and mobilization proteins, respectively. In particular, BmgA and BmgB were homologous to MocA and MocB, respectively, the two mobilization proteins of the B. fragilis mobilizable transposon Tn4399. A cis-acting origin of transfer (oriT) was localized to a 353-bp region that included nearly all of the intergenic region between bmgB and orf22 and overlapped with the 3 end of orf22. This oriT contained a putative nic site sequence but showed no significant similarity to the oriT regions of other transfer factors, including Tn4399. Despite the lack of sequence similarity between the oriTs of cLV25 and Tn4399, a mutation in the cLV25 putative DNA relaxase, bmgA, was partially complemented by Tn4399. In addition to the functional cross-reaction with Tn4399, a second distinguishing feature of cLV25 is that predicted proteins have similarity to proteins encoded not only by Tn4399 but by several Bacteroides sp. transfer factors, including NBU1, NBU2, CTnDOT, Tn4555, and Tn5520.Bacteroides fragilis is an obligate anaerobe of the colon and a significant opportunistic pathogen. Antibiotic resistance among Bacteroides spp. is rapidly increasing, largely due to the dissemination of DNA transfer factors (plasmids and transposons) harbored by members of this genus. Transfer factors can be divided into two classes, conjugative and mobilizable. Conjugative plasmids and transposons are self-transmissible, encoding both functions of DNA transfer, that is, DNA processing events for transfer initiation and mating apparatus formation, which brings donor and recipient bacteria into stable cell-cell contact. In contrast, mobilizable plasmids and transposons carry mobilization genes only for transfer initiation, utilizing a mating apparatus provided by a coresident conjugative transfer factor.All transfer factors also contain a cis-acting origin of transfer (oriT), where transfer is initiated. DNA processing events at the oriT consist of specific protein-DNA and protein-protein interactions that result in the formation of a relaxed DNAprotein complex called the relaxosome (see references 7, 9, 14, and 25 for detailed descriptions). In brief, mobilization proteins assemble at the oriT, with one protein, the DNA relaxase, nicking a single DNA strand at the nic site. The nicked DNA strand is then transferred with 5Ј-to-3Ј polarity to a recipient bacterium via a mating bridge that is...