CHARACTERIZATION OF A NEW-TYPE OF BACTEROIDES CONJUGATIVE TRANSPOSON,TC-R-EM(R)-7853

Results of previous investigations suggested that the conjugative tran sposons found in human colonic Bacteroides species were all members of a closely related family of elements, exemplified by Tc-r Em(r) DOT. We have now found a new type of conjugative transposon, Tc-r Em(r) 785 3, that does not belong to this family. Tc-r Em(r) 7853 has approximat ely the same size as the Tc-r Em(r) DOT-type elements (70 to 80 kbp) a nd also carries genes encoding resistance to tetracycline (Tc-r) and e rythromycin (Em(r)); however, it differs from previously described con jugative transposons in a number of ways. Its transfer is not regulate d by tetracycline and its transfer genes are not controlled by the reg ulatory genes rteA and rteB, which are found on Tc-r Em(r) DOT and rel ated conjugative transposons. Its ends do not cross-hybridize with the ends of Tc-r Em(r) DOT-type conjugative transposons, and the Em(r) ge ne it carries does not cross-hybridize with ermF, the Em(r) gene found on all previously studied Bacteroides conjugative transposons. There is only one region with high sequence similarity between Tc-r Em(r) 78 53 and previously characterized elements, the region that contains the Tc-r gene, tetQ. This sequence similarity ends 145 bp upstream of the start codon and 288 bp downstream from the stop codon. A 2-kbp region upstream of tetQ on Tc-r Em(r) 7853 cross-hybridized with four additi onal EcoRV fragments of Bacteroides thetaiotaomicron 7853 DNA other th an the one that contained tetQ. These additional cross-hybridizing ban ds were not part of Tc-r Em(r) 7853, but one of them cotransferred wit h Tc-r Em(r) 7853 in some matings. Thus, at least one of the additiona l cross-hybridizing bands may be associated with another conjugative e lement or with an element that is mobilized by Tc-r Em(r) 7853. DNA th at cross-hybridized with the upstream region was found in one clinical isolate of Bacteroides ovatus and four Tc-r isolates of Prevotella ru minicola.