Embedded elements in the IncPβ plasmids R772 and R906 can be mobilized and can serve as a source of diverse and novel elements

Abstract

IncP plasmids are important contributors to bacterial adaptation. Their phenotypic diversity is due largely to accessory regions located in one or two specific parts of the plasmid. The accessory regions are themselves diverse, as judged from sequenced plasmids mostly isolated from nonclinical sources. To further understand the diversity, evolutionary history and functional attributes of the accessory regions, we compared R906 and R772, focusing on the oriV-trfA accessory region. These IncPb plasmids were from porcine and clinical sources, respectively. We found that the accessory regions formed potentially mobile elements, Tn510 (from R906) and Tn511 (from R772), that differed internally but had identical borders. Both elements appeared to have evolved from a TnAO22-like mer transposon that had inserted into an ancestral IncPb plasmid and then accrued additional transposable elements and genes from various proteobacteria. Structural comparisons suggested that Tn510 (and a descendent in pB10), Tn511 and the mer element in pJP4 represent three lineages that evolved from the same widely dispersed IncPb carrier. Functional studies on Tn511 revealed that its mer module is inactive due to a merT mutation, and that its aphAI region is prone to deletion. More significantly, we showed that by providing a suitable transposase gene in trans, the defective Tn510 and Tn511 could transpose intact or in part, and could also generate new elements (stable cointegrates and novel transposons). The ingredients for assisted transposition events similar to those observed here occur in natural microcosms, providing non-self-mobile elements with avenues for dispersal to new replicons and for structural diversification. This work provides an experimental demonstration of how the complex embedded elements uncovered in IncP plasmids and in other plasmid families may have been generated.

The Tn 3 -family of Replicative Transposons

“…A similar nested organization has been reported for other phenotypically distinct Tn3family transposons such as the Yersinia enterocolitica lactose Tn951 (80) or the mercury-resistance derivatives Tn510 and Tn511 (81). Some of these are transpositiondefective and require an active copy of a related element to be mobilized in trans (46,77,80,81).…”

“…This is viewed as an adaptive strategy allowing movement and reassortment of different combinations of catabolic functions within and between separate plasmids from environmental bacteria (6). A similar nested organization has been reported for other phenotypically distinct Tn3family transposons such as the Yersinia enterocolitica lactose Tn951 (80) or the mercury-resistance derivatives Tn510 and Tn511 (81). Some of these are transpositiondefective and require an active copy of a related element to be mobilized in trans (46,77,80,81).…”

The Detection and Sequencing of a Broad-Host-Range Conjugative IncP-1β Plasmid in an Epidemic Strain of Mycobacterium abscessus subsp. bolletii

“…The Tn 501 -like mer -resistance transposons present in the pMAB01, pB10, R906, pJP4 and pAKD plasmids possess the same insertion and 5-bp direct repeat sequence (5’TGCCT3’) adjacent to the IR, suggesting that these transposons are all derived from a progenitor transposon that entered a common ancestor of these plasmids by transposition insertion [21], [24], [44]. In pMAB01, pB10 and R906, the transposase gene ( tnpA’ ) is interrupted at the same point, i.e., after 264 bp, by the insertion of a remnant copy of Tn 5393c .…”

Broad-Host-Range Plasmids from Agricultural Soils Have IncP-1 Backbones with Diverse Accessory Genes

“…Plasmid R772 was probably not formed from the pAKD29-like ancestral plasmid, since the 3Ј end of tnpA Tn501 is much longer (638 bp) and does not have an IS1071-like element near it. Formation of R772 by the insertion and subsequent disruption of the Tn501-like element by multiple Tn21-like elements has been recently hypothesized (20). Thus, although R772 is very similar to R906, pB10, and pJP4, it may have been formed through a different pathway.…”

“…The eight plasmids carrying the first type of Tn501-like transposon (all except pAKD1) are closely related to the four IncP-1␤ plasmids pB10, R906, R772, and pJP4 based not only on high nucleotide sequence identity (99 to 100%) in their backbone genes but also on the occurrence of a Tn501-like mercury transposon in exactly the same site (20,25,28). There are two possible hypotheses to explain this observation: (i) all plasmids share a recent common ancestor that already contained the Tn501-like transposon and, thus, that transposon inserted once into an ancestor of these plasmids, which subsequently gained different accessory genes, such as a Tn5393c transposon in pB10 or the tfd genes in pJP4, or (ii) the sequence between oriV and trfA in IncP-1␤ plasmids serves as a hot spot for the insertion of Tn501-like transposons, and this occurred multiple times in these seven plasmids.…”