Tetracycline Regulation of Conjugal Transfer Genes

Abstract

This chapter focuses on tetracycline-responsive regulatory system of the Bacteroides conjugative transposons. If a Bacteroides strain carrying a conjugative transposon is exposed to tetracycline, the frequency with which the conjugative transposon is transferred increases by at least 1,000-fold. The Bacteroides conjugative transposons not only transfer themselves from a donor to a recipient cell but are also capable of mobilizing coresident plasmids and excising and mobilizing unlinked elements called nonreplicating bacteroides units (NBUs). Insertional disruption of rteA or rteB abolishes element self-transfer of the conjugative transposon, mobilization of coresident plasmids, and excision-circularization and mobilization of NBUs. Two obvious candidates for genes that might be controlled by RteC are the mobilization gene(s) of the conjugative transposon, which nick the element's own oriT and initiate transfer, and the genes involved in excision. At first, RteC was assumed to be a transcriptional activator, although there was no evidence that RteC binds DNA nor was any DNA binding motif apparent in the deduced amino acid sequence of RteC. The regulatory machinery of the Bacteroides conjugative transposons is turning out to be much more complex than expected. It is important to bear in mind the fact that transfer of tetQ would not have been detected if tetracycline had not been incorporated in the medium used to grow the donors. Thus, some chromosomally encoded genes that appear to be nontransmissible could actually be carried on a conjugal element, whose transfer functions must be stimulated by some inducer not normally included in the medium.