ICETh1 & ICETh2, two mobile genetic elements coordinated in thermus thermophilus transjugation

Abstract

Horizontal Gene Transfer (HGT) is considered one of the most important sources of bacterial evolution. In the thermophilic bacterium Thermus thermophilus, the transjugation mechanism has been described as a highly efficient HGT system. This mechanism consists in the transfer of DNA from a donor to a recipient cell, being necessary the machinery for DNA donation in the donor and the transformation machinery in the recipient cell. In the strain HB27 of this organism, two small Integrative and Conjugative Elements (ICEs), have been discovered and we show how they coordinate their activities in the transjugation process. ICETh1 encodes the machinery necessary for DNA donation, where the translocase TdtA is essential, as it is probably the most important protein in this process. However, accessory proteins such as the nuclease NurA and the restrictase Tth111II are also relevant for this event, likely processing DNA prior to its transfer mediated by TdtA. Any locus in the genome can be transferred to a recipient cell; however, the ICEThs show a higher transfer rate. We have shown that ICETh1 is not capable by itself to excise or integrate in the chromosome. This process is dependent on a specialized excision/integration module encoded in ICETh2, a second mobile element that can excise and integrate both ICEThs in their respective sites despite being catalyzed by a single enzyme These ICEThs seem to exhibit a higher excision rate and apparently replicate under stress conditions produced by UV or in the absence of the primase/polymerase PrimPol encoded by ICETh2. Autonomous replication of both ICEThs, even when it is not clear, could be driven by a TOPRIM-domain homologue protein. A Toxin-Antitoxin (TA) system could assure the presence of ICETh1 in the cell population via post-segregational killing. Additionally, the possibility exists that the TOPRIM-domain homologue, encoded in ICETh2, could be required for some cellular function promoting somehow the maintenance of ICETh1 and, to a lesser extent of ICETh2. Furthermore, in this work it is proposed a retro-transfer model for DNA transjugation in which DNA fragments, apart from the ICEThs themselves, can be transferred to a recipient cell in which they integrate. Then, with the intervention of ICETh1 transjugation machinery, DNA fragments from the recipient cell could be transferred back in the opposite direction to the original donor, followed by integration, generating mosaicism in the progeny.