Characterization of a Bacteroides mobilizable transposon, NBU2, which carries a functional lincomycin resistance gene

The mobilizable Bacteroides element NBU2 (11 kbp) was found originally in t wo Bacteroides clinical isolates, Bacteroides fragilis ERL and B. thetaiota omicron DOT. At first, NBU2 appeared to be very similar to another mobiliza ble Bacteroides element, NBU1, in a 2.5-kbp internal region, but further ex amination of the full DNA sequence of NBU2 now reveals that the region of n ear identity between NBU1 and NBU2 is limited to this small region and that , outside this region, there is little sequence similarity between the two elements. The integrase gene of NBU2, intN2, was located at one end of the element. This gene was necessary and sufficient for the integration of NBU2 , The integrase of NBU2 has the conserved amino acids (R-H-R-Y) in the C-te rminal end that are found in members of the lambda family of site-specific integrases, This was also the only region in which the NBU1 and NBU2 integr ases shared any similarity (28% amino acid sequence identity and 49% sequen ce similarity), Integration of NBU2 was site specific in Bacteroides specie s. Integration occurred in two primary sites in B. thetaiotaomicron, Both o f these sites were located in the 3' end of a serine-tRNA gene NBU2 also in tegrated in Escherichia coli, but integration was much less site specific t han in B. thetaiotaomicron. Analysis of the sequence of NBU2 revealed two p otential antibiotic resistance genes. The amino acid sequences of the putat ive proteins encoded by these genes had similarity to resistances found in gram-positive bacteria. Only one of these genes was expressed in B. thetaio taomicron, the homolog of linA, a lincomycin resistance gene from Staphyloc occus aureus. To determine how widespread elements related to NBU1 and NBU2 are in Bacteroides species, we screened 291 Bacteroides strains, Elements with some sequence similarity to NBU2 and NBU1 were widespread in Bacteroid es strains, and the presence of linA, in Bacteroides strains was highly cor related with the presence of NBU2, suggesting that NBU2 has been responsibl e for the spread of this gene among Bacteroides strains. Our results sugges t that the NBU-related elements form a large and heterogeneous family, whos e members have similar integration mechanisms but have different target sit es and differ in whether they carry resistance genes.