Regulation of CTnDOT conjugative transfer is a complex and highly coordinated series of events

Abstract

CTnDOT is a 65-kb conjugative transposon that is found in Bacteroides spp., which are one of the more abundant members within the lower human gastrointestinal tract. CTnDOT encodes resistance to the antibiotics erythromycin and tetracycline (Tc). An interesting feature of CTnDOT is that exposure to low levels of Tc induces a cascade of events that ultimately results in CTnDOT conjugative transfer. However, Tc is apparently not a switch that activates transfer but rather a signal that appears to override a series of negative regulators that inhibit premature excision and transfer of CTnDOT. In this minireview, we summarize over 20 years of research that focused on elucidating the highly coordinated regulation of excision, mobilization, and transfer of CTnDOT.

Importance: Bacteroides spp. are abundant commensals in the human colon, but they are also considered opportunistic pathogens, as they can cause life-threatening infections if they should escape the colon. Bacteroides spp. are the most common cause of anaerobic infections and are rather difficult to treat due to the prevalence of antibiotic resistance within this genus. Today over 80% of Bacteroides are resistant to tetracycline (Tc), and a study looking at both clinical and community isolates demonstrated that this resistance was specifically due to the conjugative transposon CTnDOT.

FIG 1

Overview of the CTnDOT life cycle. CTnDOT is a 65-kb conjugative transposon present in Bacteroides spp. that encodes resistance to the antibiotics erythromycin and tetracycline. The conjugative transfer of CTnDOT is stimulated by tetracycline induction. The first step in conjugative transfer is that CTnDOT must first excise from the host chromosome to form a circular intermediate. CTnDOT is then nicked at the oriT after which CTnDOT is then replicated and made double-stranded before integrating into the chromosomes of both the donor and recipient.

FIG 2

Positive regulation of CTnDOT upon exposure to tetracycline. In step 1, the tetQ-rteA-rteB operon is regulated by a translational attenuation mechanism where after exposure to tetracycline (+ Tc), translation of this region resumes; translation of this region ultimately allows for RteB to activate the transcription of RteC (step 2). RteC then activates transcription of the excision operon (step 3). Proteins encoded by the excision operon not only excise CTnDOT from the chromosome but are also involved in transcriptional regulation (step 4). Xis2c and Xis2d are involved in regulation of the tra region, whereas Xis2d and Exc are involved in enhancement of mob transcription. We have yet to identify any functional role for Orf3.