Evolutionary Competitiveness of Two Natural Variants of the IncQ-Like Plasmids, pRAS3.1 and pRAS3.2

Abstract

Plasmids pRAS3.1 and pRAS3.2 are natural variants of the IncQ-2 plasmid family, that except for two differences, have identical plasmid backbones. Plasmid pRAS3.1 has four 22-bp iterons in its oriV region, while pRAS3.2 has only three 6-bp repeats and pRAS3.1 has five 6-bp repeats in the promoter region of the mobB-mobA/repB genes and pRAS3.2 has only four. In previous work, we showed that the overall effect of these differences was that when the plasmid was in an Escherichia coli host, the copy numbers of pRAS3.1 and pRAS3.2 were approximately 41 and 30, respectively. As pRAS3.1 and pRAS3.2 are likely to have arisen from the same ancestor, we addressed the question of whether one of the variants had an evolutionary advantage over the other. By constructing a set of identical plasmids with the number of 22-bp iterons varying from three to seven, it was found that plasmids with four or five iterons displaced plasmids with three iterons even though they had lower copy numbers. Furthermore, the metabolic load that the plasmids placed on E. coli host cells compared with plasmid-free cells increased with copy number from 10.9% at a copy number of 59 to 2.6% at a copy number of 15. Plasmid pRAS3.1 with four 22-bp iterons was able to displace pRAS3.2 with three iterons when both were coresident in the same host. However, the lower-copy-number pRAS3.2 placed 2.8% less of a metabolic burden on an E. coli host population, and therefore, pRAS3.2 has a competitive advantage over pRAS3.1 at the population level, as pRAS3.2-containing cells would be expected to outgrow pRAS3.1-containing cells.Plasmids of the IncQ family are characterized by their relatively small size (ϳ6 to 15 kb), their ability to replicate in a very wide range of bacterial host cells, and by being readily mobilizable by certain self-transmissible plasmids, in particular the broad-host-range IncP plasmids such as RP4 or RK2 (22). IncQ family plasmids have been subdivided into the IncQ-1 and IncQ-2 groups depending on whether their mobilization genes are of the three-gene IncQ type or the five-gene IncP type. Both subgroups of IncQ family plasmids have similar replicons that consist of three genes that encode a helicase (repA), a primase (repB), a DNA-binding initiator protein (repC), and an oriV region. The oriV region typically contains three complete, identical 22-bp iterons (or 20-bp iterons with 2-bp spacers) that serve as the binding site for RepC proteins (18,22). Although some IncQ family plasmids may possess more than three copies of the iterons, the additional iterons are either partial copies, have sequence variations, or lack the spacer regions (22). It has been shown that even a single-basepair replacement in a single iteron could result in a nonfunctional iteron-containing region and the inability of the IncQ-1 plasmid RSF1010 to replicate (19). Furthermore, in IncQ plasmids, the iterons have been shown to serve as the primary incompatibility determinants with cloned iterons on their own being able to displace the plasmid from which they were ...

High Prevalence of Plasmid-Mediated Quinolone Resistance and IncQ Plasmids Carrying qnrS2 Gene in Bacteria from Rivers near Hospitals and Aquaculture in China

“…Iterons were the major incompatibility determinants in IncQ plasmids, so point mutations within the three 20-bp conserved repeats of iterons could result in an inability or significantly reduced ability to exert incompatibility, initiate replication, or transfer genes by conjugation [ 43 ]. Meanwhile, the iteron number had a negative effect on plasmid copy number, as well as the antibiotic resistance level, and imposed metabolic burden [ 44 ]. Hence, variations in the iteron sequence may reflect the need for balance between high antibiotic resistance and a stable existence [ 45 ].…”

Detection of Variants of the pRAS3, pAB5S9, and pSN254 Plasmids in Aeromonas salmonicida subsp. salmonicida: Multidrug Resistance, Interspecies Exchanges, and Plasmid Reshaping

“…Like pRAS3.1 and pRAS3.2, pRAS3.3 has the same two ORFs that code for an active toxin-antitoxin system related to the PemK/ Mazf family (32,33).…”

The influence of the accessory genome on bacterial pathogen evolution

“…27 Another study found that an IncQ plasmid has evolved to a lower copy number variant; this variant is more competitive within the bacterial population than a higher copy number variant because it places a lower metabolic load on the bacterial host. 28 Likewise, transfer of the symbiotic plasmid from the β-rhizobium Cupriavidus taiwanensis to the taxonomically related plant pathogen Ralstonia solanacearum allowed the adaptive evolution of the latter, after the inactivation of the T3SS by a single or a double mutation, into a nodulating symbiont. 29 Although these data resulted from artificial evolution settings, the experiments underscore the amazing plasticity of bacteriaplasmid associations and the potential for rapidly generating new phenotypes.…”