Abstract
BackgroundThe repABC plasmid family, which is extensively present within Alphaproteobacteria, and some secondary chromosomes of the Rhizobiales have the particular feature that all the elements involved in replication and partitioning reside within one transcriptional unit, the repABC operon. Given the functional interactions among the elements of the repABC operon, and the fact that they all reside in the same operon, a common evolutionary history would be expected if the entire operon had been horizontally transferred. Here, we tested whether there is a common evolutionary history within the repABC operon. We further examined different incompatibility groups in terms of their differentiation and degree of adaptation to their host.ResultsWe did not find a single evolutionary history within the repABC operon. Each protein had a particular phylogeny, horizontal gene transfer events of the individual genes within the operon were detected, and different functional constraints were found within and between the Rep proteins. When different repABC operons coexisted in the same genome, they were well differentiated from one another. Finally, we found different levels of adaptation to the host genome within and between repABC operons coexisting in the same species.ConclusionHorizontal gene transfer with conservation of the repABC operon structure provides a highly dynamic operon in which each member of this operon has its own evolutionary dynamics. In addition, it seems that different incompatibility groups present in the same species have different degrees of adaptation to their host genomes, in proportion to the amount of time the incompatibility group has coexisted with the host genome.
Highlights
The repABC plasmid family, which is extensively present within Alphaproteobacteria, and some secondary chromosomes of the Rhizobiales have the particular feature that all the elements involved in replication and partitioning reside within one transcriptional unit, the repABC operon
We found different levels of adaptation to the host genome within and between repABC operons coexisting in the same species
The collection of homologous repABC operons To date, at least 81 repABC operons have been recognized across the class Alphaproteobacteria [1]
Summary
The repABC plasmid family, which is extensively present within Alphaproteobacteria, and some secondary chromosomes of the Rhizobiales have the particular feature that all the elements involved in replication and partitioning reside within one transcriptional unit, the repABC operon. In contrast to other low copy-number plasmids, in which the elements involved in plasmid replication and segregation are located on different loci (each one under its own regulatory circuit), the repABC plasmids contain all the elements required for replication and partition within the repABC operon This transcriptional unit comprises three proteinencoding genes (repA, repB, and repC) and a gene encoding a small antisense RNA (ctRNA) [5], which is located within the repB-repC intergenic region. The third protein-encoding gene of the operon, repC, is essential for plasmid replication; it encodes the initiator protein, RepC, which exerts its function by binding the origin of replication located within its own coding sequence [1,2,6] Taking these observations into account, it is reasonable to hypothesize that the repABC operon is under concerted evolutionary pressures aimed at maintaining its functionality and avoiding incompatibility with other repABC operons. The second chromosomes of Agrobacterium vitis S4 and Agrobacterium tumefaciens C58 have a repABC origin of replication [7]
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