Many strains of human colonic Bacteroides carry conjugal tetracycline resistance (Tcr) elements. Some transfer Tcr only, whereas others co- transfer Tcr and a gene that codes for resistance to erythromycin (Emr) The same gene that codes for Emr also codes for resistance to clindamycin, one of the few antibiotics that is effective for treating Bacteroides infections. The Bacteroides Tcr and TcrEmr elements are thought to be located in the chromosome rather than carried on plasmids. An unusual feature of most of these elements is that both the frequency of transfer and the level of resistance are regulated by tetracycline. Recently, we have also shown that some of the elements mediate the tetracycline-dependent appearance of plasmid-like forms, an activity that could be associated with excision and transfer of the element itself. Using a n E. coli-Bacteroides shuttle cosmid we constructed, we have cloned portions of a Bacteroides TcrEmr element. Our clones contain genes for all known activities of the element. These clones make it possible for the first time to investigate the conjugal Bacteroides Tcr elements at the molecular level. The long term goal of this project is to determine the gene organization of the element, how the Tcr, transfer and excisase genes are regulated, what steps are involved in excision and reintegration of the element in the chromosome, and whether the Tcr elements found in different Bacteroides clinical isolates are closely related. We will also continue to investigate an unusual tetracycline resistance ( which was found in Bacteroides but which works only in aerobically grown E. coli. Recently we have shown that this gene codes for a protein that inactivates tetracycline. We will try to identify the inactivated form of tetracycline. We will also sequence the gene and its upstream region. Although expression of the gene is not regulated by tetracycline, deletions in the upstream region affect the level of resistance. Information from the sequence may help to resolve the role of this region.
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