An unusual type of cointegrate formation between a Bacteroides plasmid and the excised circular form of an integrated element (NBU1).

Evidence for an unusual type of cointegrate formation was found as the result of analyzing three integration events that fused a mobilization-deficient Bacteroides plasmid (pEG920) with the excised circular form of a nonreplicating Bacteroides element (NBU1). NBU1 is capable of inserting itself into DNA segments, but the cointegrates were the result of invasion of NBU1 by pEG920, not vice versa. The same site on pEG920 was involved in all cases. Sequence analysis of the cointegrates suggested that the integration events may have been the result of a multistep process in which a conjugative transposon was involved.