EXCISION, TRANSFER, AND INTEGRATION OF NBU1, A MOBILIZABLE SITE-SELECTIVE INSERTION ELEMENT

The Bacteroides species harbor a family of conjugative transposons cal led tetracycline resistance elements (Tc(r) elements) that transfer th emselves from the chromosome of a donor to the chromosome of a recipie nt, mobilize coresident plasmids, and also mediate the excision and ci rcularization of members of a family of 10- to 12-kbp insertion elemen ts which share a small region of DNA homology and are called NBUs (for nonreplicating Bacteroides units). The NBUs are sometimes cotransferr ed with Tc(r) elements, and it was postulated previously that the exci sed circular forms of the NBUs were plasmidlike forms and were transfe rred like plasmids and then integrated into the recipient chromosome. We used chimeric plasmids containing one of the NBUs, NBU1, and a Bact eroides-Escherichia coli shuttle vector to show that this hypothesis i s probably correct. NBU1 contained a region that allowed mobilization by both the Tc(r) elements and IncP plasmids, and we used these conjug al elements to allow us to estimate the frequencies of excision, mobil ization, and integration of NBU1 in Bacteroides hosts to be approximat ely 10(-2), 10(-5) to 10(-4), and 10(-2), respectively. Although funct ions on the Tc(r) elements were required for the excision-circularizat ion and mobilization of NBU1, no Tc(r) element functions were required for integration into the recipient chromosome. Analysis of the DNA se quences at the integration region of the circular form of NBU1, the pr imary insertion site in the Bacteroides thetaiotaomicron 5482 chromoso me, and the resultant NBU1-chromosome junctions showed that NBU1 appea red to integrate into the primary insertion site by recombining within an identical 14-bp sequence present on both NBU1 and the target, thus leaving a copy of the 14-bp sequence at both junctions. The apparent integration mechanism and the target selection of NBU1 were different from those of both XBU4422, the only member of the conjugal Tc(r) elem ents for which these sequences are known, and Tn4399, a mobilizable Ba cteroides transposon. The NBUs appear to be a distinct type of mobiliz able insertion element.