Transfer region of a Bacteroides conjugative transposon contains regulatory as well as structural genes.

Conjugative transposons (CTns) are integrated elements that excise themselves from the chromosome to form a circular transfer intermediate that is transferred by conjugation to a recipient. In an earlier paper, the excision step was shown to be regulated by tetracycline and to be dependent on the regulatory gene, rteC. In this paper, we report that genes involved in conjugal transfer are also regulated by tetracycline but that regulation is more complex. Genes contained within a 20-kbp region that is sufficient for conjugal transfer were disrupted by single crossover integration events. Most of the disruptions abolished transfer of the CTn. None of them abolished excision. Antibodies to two of the proteins encoded in this region (TraG and TraN) were obtained and used to show that production of these proteins was dependent on tetracycline stimulation. Both TraG and TraN were membrane proteins. A surprising finding was that a disruption in the gene traQ increased transfer of CTnERL over 100-fold. Thus, TraQ may be a repressor protein that controls expression of transfer genes. If so, TraQ is not the only protein that controls expression of transfer genes because production of TraG and TraN in the traQ disruption mutant was still dependent on tetracycline stimulation.

Characterization of the 13-kilobase ermF region of the Bacteroides conjugative transposon CTnDOT.

The conjugative transposon CTnDOT is virtually identical over most of its length to another conjugative transposon, CTnERL, except that CTnDOT carries an ermF gene that is not found on CTnERL. In this report, we show that the region containing ermF appears to consist of a 13-kb chimera composed of at least one class I composite transposon and a mobilizable transposon (MTn). Although the ermF region contains genes also carried on Bacteroides transposons Tn4351 and Tn4551, it does not contain the IS4351 element which is found on these transposons. In CTnDOT, insertion of the ermF region occurred near a stem-loop structure at the end of orf2, an open reading frame located immediately downstream of the integrase (int) gene of CTnDOT, and in a region known to be important for excision of CTnERL and CTnDOT. The chimera that comprises the ermF region can apparently no longer excise and circularize, but it contains a functional mobilization region related to that described for the Bacteroides MTn Tn4399. Analysis of 19 independent Bacteroides isolates showed that the ermF region is located in the same position in all of the strains analyzed and that the compositions of the ermF region are almost identical in these strains. Therefore, it appears that CTnDOT-like elements present in community and clinical isolates of Bacteroides were derived from a common ancestor and proliferated in the diverse Bacteroides population.