Horizontal gene transfer in Thermus thermophilus: mechanisms and barriers

Resumen

Comparative genome analysis have evidenced that fluent genetic exchange is mainly mediated by Horizontal Gene Transfer (HGT), thus recognized as a leading force of prokaryotic evolution and microbial genetic diversity. Transduction, transformation and conjugation are the three standard mechanisms driving HGT. The ancient thermophilic bacteria Thermus thermophilus laterally transfers DNA in a really efficient way due to a highly sophisticated competence machinery as well as by a conjugation-like process. However, conjugation in T. thermophilus emerges rather unconventional as it is efficient between isogenic cells, thus, bidirectional, and no type IV secretion systems seems to be involved. Besides, genes associated to the megaplasmid are transferred with higher frequencies (~10 fold) than those localized in the chromosome. No evident order in the transfer among megaplasmid markers contrasts with the identification of multiple hotspots of transfer in the chromosome. Several of these loci showing higher transfer frequencies are encoded near putative Tth111 type II restriction sites which might work as OriTs. The competence machinery actively participates in conjugation, being required in the receptor cell but not in the donor. Hence, a two-step model (push-pull) is proposed, where the donor energetically pumps DNA to a receptor cell, which actively pulls in the DNA transferred with its competence apparatus. Two active hexameric ATPases named CptA and HerA, paralogs to the helicases HerA and FtsK from other bacteria, have been identified as chief components of the pushing step. Therefore, T. thermophilus is able to proficiently exchange DNA by transformation (including DNA-protected vesicles which could work as long distance vehicles), and largely by conjugation. This fruitful dynamic gene flow is compatible with a battery of protective strategies to prevent potentially harmful invasion of genetic parasites. Among them, the Argonaute protein elicits a DNA-DNA interference on DNA taken up by transformation. I proved larger insights of Argonaute-mediated interference, acting, in vivo, against virtually any kind of DNA template. However, when such DNA was transferred by a conjugation-like process, Argonaute was not activated, suggesting a selective immunity towards the way in which DNA is acquired. This fact, together with the higher efficiency shown by conjugation compared to natural competence when the same DNA was transferred, enforced the proposal of conjugation as the major motor of shared traits among populations of Thermus spp in thermal environments.