Genes involved in production of plasmidlike forms by a Bacteroides conjugal chromosomal element share amino acid homology with two-component regulatory systems

Abstract

Many human colonic Bacteroides strains carry large (greater than 70-kbp) self-transmissible chromosomal tetracycline resistance (Tcr) elements. These Tcr elements can also mediate the excision and circularization of discrete nonadjacent segments of chromosomal DNA which are designated NBUs (nonreplicating Bacteroides units). We have localized a 6.5-kbp segment of Tcr element DNA that mediates NBU excision and circularization. Analysis of the DNA sequence of this region indicated that it contained three open reading frames, all transcribed in the same direction. The first gene was the Tcr gene, tetQ. The second two open reading frames exhibited amino acid similarity to known two-component regulatory systems. Complementation and gene fusion data supported the hypothesis that the three genes were organized in an operon. Transcription from the tetQ promoter region was inducible by tetracycline, as might be expected from the previous finding that NBU excision was detectable only in cells preexposed to tetracycline. The 6.5-kbp region appeared to be essential not only for NBU excision but also for self-transfer of the elements, another activity that is enhanced by preexposure to tetracycline. Accordingly, the two genes downstream of tetQ have been designated rteA and rteB (regulation of Tcr elements).