The broad-host-range IncP beta plasmid R751 can mobilize itself from Escherichia coli to Bacteroides spp, but it is not maintained in Bacteroides spp. If R751 carries the Bacteroides transposon Tn4351, it can be integrated into the Bacteroides chromosome. Previously we showed that R751, integrated in the chromosome of Bacteroides uniformis, cannot mobilize itself out of B. uniformis into E. coli or isogenic B. uniformis strains. In this report, we showed that if the Bacteroides conjugative tetracycline resistance element Tcr ERL was coresident with the R751 insertion in B. uniformis, derivatives of R751 were transferred to E. coli, where they were recovered as plasmids. The most common derivatives were R751::Tn4351 and R751::IS4351, but some strains transferred R751 derivatives, containing additional DNA segments ranging in size from 10 to 23 kilobases. These DNA inserts cross-hybridized with chromosomal DNA from B. uniformis which did not carry the Tcr ERL element. Therefore, the inserts appeared to be segments of the wild-type B. uniformis chromosome and were not associated with the Tcr ERL element. The transfer of integrated R751 from B. uniformis was independent of the RecA phenotype of the E. coli recipients and did not appear to be due to transfer of B. uniformis chromosomal DNA, followed by RecA-dependent recombination between homologous IS4351 sequences to form the resultant R751 plasmid derivatives. Consistent with this, no transfer of Tn4351 (associated with the cointegrated R751) from B. uniformis donors to isogenic B. uniformis recipients was detected (< 10(-8)). Our data support the hypothesis that R751 excises from the B. uniformis chromosome by recombination involving flanking Tn4351 or IS4351 sequences and forms nonreplicating circles. The mobilization of these circular forms out of B. uniformis to E.coli is then facilitated by the Tcr ERL element.