Region-Specific Insertion of Transposons in Combination with Selection for High Plasmid Transferability and Stability Accounts for the Structural Similarity of IncP-1 Plasmids

Abstract

The overall architecture of IncP-1 plasmids is very conserved in that the accessory genes are typically located in one or two specific regions: between oriV and trfA and between the tra and trb operons. Various hypotheses have been formulated to explain this, but none have been tested experimentally. We investigated whether this structural similarity is due to region-specific transposition alone or also is reliant on selection for plasmids with insertions limited to these two regions. We first examined the transposition of Tn21Km into IncP-1␤ plasmid pBP136 and found that most Tn21Km insertions (67%) were located around oriV. A similar experiment using the oriV region of IncP-1␤ plasmid pUO1 confirmed these results. We then tested the transferability, stability, and fitness cost of different pBP136 derivatives to determine if impairment of these key plasmid characters explained the conserved plasmid architecture. Most of the pBP136 derivatives with insertions in transfer genes were no longer transferable. The plasmids with insertions in the oriV-trfA and tra-trb regions were more stable than other plasmid variants, and one of these also showed a significantly lower fitness cost. In addition, our detailed sequence analysis of IncP-1 plasmids showed that Tn402/5053-like transposons are situated predominantly between the tra and trb operons and close to the putative resolution site for the ParA resolvase, a potential hot spot for those transposons. Our study presents the first empirical evidence that region-specific insertion of transposons in combination with selection for transferable and stable plasmids explains the structural similarity of IncP-1 plasmids.

Tn502and Tn512AreresSite Hunters That Provide Evidence of Resolvase-Independent Transposition to Random Sites

“…However, exceptions exist among naturally occurring elements, for example, Tn402 in R751 (36) and Tn4672 in pUO1 (43) (both IncP␤ plasmids) and In5 in pSCH884 (6, 13) (Inc type unknown). In the IncP␤ plasmids, the transposon occurs in a region that corresponds to the par locus of RP1 (44), suggesting that insertion occurred originally via the pardependent process. Our observations that Tn502 can sometimes insert in the vicinity of the res target region in pUB307, rather than within it (Fig.…”

“…4). Sequence comparisons with the intact res-parA backbone region of pBP136, which lacks any inserts (44), suggest that inheritance of the Tn5053/Tn402-like elements resulted in deletions in the IncP␤ plasmids of 0.4 to 2.2 kb (Fig. 4).…”

The Behavior and Significance of Degradative Plasmids Belonging to Inc Groups in <i>Pseudomonas</i> within Natural Environments and Microcosms

“…The core regions of IncP-1 group plasmids show high identity. Accessory genes are usually carried by a transposon inserted in two regions, first between the oriV and the trfA genes (involved in replication initiation), and second, between the tra (Dtr) and trb (Mpf) operons (1,86,102). This suggests that IncP-1 is one of the vectors responsible for the dissemination of accessory genes among environmental bacteria.…”

Spatial structure and nutrients promote invasion of IncP-1 plasmids in bacterial populations

“…To determine plasmid cost, plasmid-free strains were competed against their plasmid-bearing counterparts (either pB10 or pB10::rfp). Competition experiments were performed as described previously (Bouma and Lenski, 1988;Sota et al, 2007) and the plasmid cost (c) was calculated as 1ÀW, where W is the relative fitness (Heuer et al, 2007).…”