CHARACTERIZATION OF THE MOBILIZATION REGION OF A BACTEROIDES INSERTION ELEMENT (NBU1) THAT IS EXCISED AND TRANSFERRED BY BACTEROIDES CONJUGATIVE TRANSPOSONS

Many Bacteroides clinical isolates carry large conjugative transposons that, in addition to transferring themselves, excise, circularize, an d transfer smaller, unlinked chromosomal DNA segments called NBUs (non replicating Bacteroides units). We report the localization and DNA seq uence of a region of one of the NBUs, NBU1, that was necessary and suf ficient for mobilization by Bacteroides conjugative transposons and by IncP plasmids. The fact that the mobilization region was internal to NBU1 indicates that the circular form of NBU1 is the form that is mobi lized. The NBU1 mobilization region contained a single large (1.4-kbp) open reading frame (ORFI), which was designated mob. The oriT was loc ated within a 220-bp region upstream of mob. The deduced amino acid se quence of the mob product had no significant similarity to those of mo bilization proteins of well-characterized Escherichia coli group plasm ids such as RK2 or of either of the two mobilization proteins of Bacte roides plasmid pBFTM10. There was, however, a high level of similarity between the deduced amino acid sequence of the mob product and that o f the product of a Bacteroides vulgatus cryptic open reading frame clo sely linked to a cefoxitin resistance gene (cfxA).