The Integrative Conjugative Element (ICE) ofMycoplasma agalactiae: key elements involved in horizontal dissemination and influence of co-resident ICEs

Abstract

ABSTRACTThe discovery of integrative conjugative elements (ICEs) in wall-less mycoplasmas and the demonstration of their role in massive gene flows within and across species has shed new light on the evolution of these minimal bacteria. Of these, ICEA of the ruminant pathogenMycoplasma agalactiaerepresents a prototype and belongs to a new clade of the Mutatorlike superfamily that has no preferential insertion site and often occurs as multiple chromosomal copies. Here, functional genomics and mating experiments were combined to address ICEA functions and define the minimal ICEA chassis conferring conjugative properties toM. agalactiae. Data further indicated a complex interaction among co-resident ICEAs, since the minimal ICEA structure was influenced by the occurrence of additional ICEA copies that cantrans-complement conjugative-deficient ICEAs. However, this cooperative behavior was limited to the CDS14 surface lipoprotein, which is constitutively expressed by co-resident ICEAs, and did not extend to other ICEA proteins including thecis-acting DDE recombinase and components of the mating channel whose expression was only sporadically detected. Remarkably, conjugative-deficient mutants containing a single ICEA copy knocked-out incdsl4can be complemented by neighboring cells expressing CDS14. This result, together with the conservation of CDS14 functions in closely related species, may suggest a way for mycoplasma ICEs to extend their interaction outside of their chromosomal environment. Overall, this study provides a first model of conjugative transfer in mycoplasmas and offers valuable insights towards the understanding of horizontal gene transfer in this highly adaptive and diverse group of minimal bacteria.IMPRTANCEIntegrative conjugative elements (ICEs) are self-transmissible mobile genetic elements that are key mediators of horizontal gene flow in bacteria. Recently, a new category of ICEs has been identified that confer conjugative properties to mycoplasmas, a highly adaptive and diverse group of wall-less bacteria with reduced genomes. Unlike classical ICEs, these mobile elements have no preferential insertion specificity and multiple mycoplasma ICE copies can be found randomly integrated into the host chromosome. Here, the prototype ICE ofMycoplasma agalactiaewas used to define the minimal conjugative machinery and propose the first model of ICE transfer in mycoplasmas. This model unveils the complex interactions taking place among co-resident ICEs and suggests a way for these elements to extend their influence outside of their chromosomal environment. These data pave the way for future studies aiming at deciphering chromosomal transfer, an unconventional mechanism of DNA swapping that has been recently associated with mycoplasma ICEs.