TETRACYCLINE REGULATION OF GENES ON BACTEROIDES CONJUGATIVE TRANSPOSONS

Human colonic Bacteroides species harbor a family of large conjugative transposons, called tetracycline resistance (Tc(r)) elements. Activit ies of these elements are enhanced by pregrowth of bacteria in medium containing tetracycline, indicating that at least some Tc(r) element g enes are regulated by tetracycline. Previously, we identified a centra l regulatory locus on the Tc(r) elements that contained two genes, rte A and rteB, which appeared to encode a two-component regulatory system (A. M. Stevens, J. M. Sanders, N. B. Shoemaker, and A. A. Salyers, J. Bacteriol. 174:2935-2942, 1992). In the present study, we describe a gene which is located downstream of rteB in a separate transcriptional unit and which requires RteB for expression. Sequence analysis of thi s gene showed that it encoded a 217-amino-acid protein, which had no s ignificant sequence similarity to any proteins in the GenBank or EMBL data base. An insertional disruption in the gene abolished self-transf er of the Tc(r) element to Bacteroides recipients, indicating that the gene was essential for self-transfer. The disruption also affected mo bilization of coresident plasmids. Mobilization frequency was reduced 100- to 1,000-fold if the recipient was Escherichia coli but was not a ffected to the same extent if the recipient was an isogenic Bacteroide s strain. The complex phenotype of the disruption mutant suggested tha t the newly identified gene, like rteA and rteB, had a regulatory func tion. Accordingly, it has been designated rteC. Our results indicate t hat regulation of Tc(r) element functions is unexpectedly complex and may involve a cascade of regulators, with RteA and RteB exerting centr al control over secondary regulators like RteC, which in turn control subsets of Tc(r) element structural genes.