Chromosomal Transfers in Mycoplasmas: When Minimal Genomes Go Mobile

Abstract

Horizontal gene transfer (HGT) is a main driving force of bacterial evolution and innovation. This phenomenon was long thought to be marginal in mycoplasmas, a large group of self-replicating bacteria characterized by minute genomes as a result of successive gene losses during evolution. Recent comparative genomic analyses challenged this paradigm, but the occurrence of chromosomal exchanges had never been formally addressed in mycoplasmas. Here, we demonstrated the conjugal transfer of large chromosomal regions within and among ruminant mycoplasma species, with the incorporation of the incoming DNA occurring by homologous recombination into the recipient chromosome. By combining classical mating experiments with high-throughput next-generation sequencing, we documented the transfer of almost every position of the mycoplasma chromosome. Mycoplasma conjugation relies on the occurrence of an integrative conjugative element (ICE) in at least one parent cell. While ICE propagates horizontally from ICE-positive to ICE-negative cells, chromosomal transfers (CTs) occurred in the opposite direction, from ICE-negative to ICE-positive cells, independently of ICE movement. These findings challenged the classical mechanisms proposed for other bacteria in which conjugative CTs are driven by conjugative elements, bringing into the spotlight a new means for rapid mycoplasma innovation. Overall, they radically change our current views concerning the evolution of mycoplasmas, with particularly far-reaching implications given that over 50 species are human or animal pathogens.

Integrative Conjugative Elements Are Widespread in Field Isolates of Mycoplasma Species Pathogenic for Ruminants

“…ICEs are a diverse group of mobile genetic elements found in a wide range of bacteria (20) and were recently suspected to be key players in M. agalactiae HGT (19,22). Prior to this study, such genetic elements had been described in only a limited number of mycoplasma strains, raising the issue of their occurrence within species and, in turn, of their biological significance.…”

“…Indeed, our team has demonstrated the existence of chromosomal transfers from ICE-negative to ICE-positive M. agalactiae strains that occur independently of ICE movement but take advantage of the ICE conjugative machinery (19). Mycoplasma ICEs are thus essential contributors to genome plasticity, as all parts of the genome were shown to be similarly transmissible (22).…”

“…Demonstration of ICEA 5632 trafficking in current mycoplasmas is an important step toward the understanding of HGT and of the contribution of ICE in this phenomenon. Indeed, ICEA 5632 also provided the mycoplasma cell with a means for genome-wide chromosomal transfers that rely on an ICE-encoded conjugative bridge, although they are independent from ICE transfer per se (19,22).…”

Mycoplasma agalactiae Secretion of β-(1→6)-Glucan, a Rare Polysaccharide in Prokaryotes, Is Governed by High-Frequency Phase Variation

“…PCRs were performed using Phusion High-Fidelity DNA polymerase (New England BioLabs). Transformation of M. agalactiae with DNA constructions was obtained with polyethylene glycol 8000 (27).…”

Molecular biology of mycoplasmas: from the minimum cell concept to the artificial cell

“…However, this data indicates that, in addition to being able to survive against the host's defenses, mollicutes are also capable of promoting chromosomic exchange to a certain degree, which avoids genome stagnation, making possible the continued adaptation to new sites and even new hosts (Sirand-Pugnet et al 2007). Nevertheless, the mechanism by which gene transfer occurs between mollicute cells is still not well documented (Dordet-Frisoni et al 2014). …”