Abstract
Conjugation is a key mechanism of bacterial evolution that involves mobile genetic elements. Recent findings indicated that the main actors of conjugative transfer are not the well-known conjugative or mobilizable plasmids but are the integrated elements. This paper reviews current knowledge on “integrative and mobilizable elements” (IMEs) that have recently been shown to be highly diverse and highly widespread but are still rarely described. IMEs encode their own excision and integration and use the conjugation machinery of unrelated co-resident conjugative element for their own transfer. Recent studies revealed a much more complex and much more diverse lifecycle than initially thought. Besides their main transmission as integrated elements, IMEs probably use plasmid-like strategies to ensure their maintenance after excision. Their interaction with conjugative elements reveals not only harmless hitchhikers but also hunters that use conjugative elements as target for their integration or harmful parasites that subvert the conjugative apparatus of incoming elements to invade cells that harbor them. IMEs carry genes conferring various functions, such as resistance to antibiotics, that can enhance the fitness of their hosts and that contribute to their maintenance in bacterial populations. Taken as a whole, IMEs are probably major contributors to bacterial evolution.
Highlights
Conjugation is a key mechanism of horizontal gene transfer between bacteria and a major contributor of plasticity and evolution of prokaryotic genomes [1,2]
The conjugative plasmids have been discovered a long time ago and are extensively studied since (i) they are identified; (ii) they are widely distributed in bacteria and (iii) they massively contribute to the dissemination of pathogenesis and antibiotic resistance determinants
The phylogenetic analyses of the relaxases and coupling protein (CP) reveal losses, acquisitions and replacements of TcpA genes between integrative and mobilizable elements” (IMEs). It seems probable, that, like mating pair formation system (MPF) subunits encoded by the MGI SGI1, the CPs encoded by these IMES would be dispensable for their mobilization but that the replacement of the CP of the helper conjugative element by the one of the IME would promote their efficient mobilization, probably at the expense of the helper element
Summary
Conjugation is a key mechanism of horizontal gene transfer between bacteria and a major contributor of plasticity and evolution of prokaryotic genomes [1,2]. The mechanism of trans-mobilization is well documented for plasmids from Proteobacteria Mobilizable plasmids carry their own oriT, relaxase gene and frequently gene(s) encoding relaxosome accessory factor(s) but lack genes required for T4SS formation. All the very few documented IMEs discovered before 2005 display mobilization mechanisms similar to those of canonical mobilizable plasmids They carry an oriT, encode a canonical relaxase and possibly other proteins of the relaxasome but no protein of the. Starting from 2010, various non-canonical IMEs have been discovered, such as elements devoid of any relaxase, elements encoding some T4SS proteins that reshape the mating apparatus of conjugative plasmids to promote their own transfer, or putative elements that encode a non-canonical.
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