The biology, diversity and evolution of the broad host-range, promiscuous INCQ plasmids, with an emphasis on the INCQ2 sub-family

[摘要] ENGLISH ABSTRACT: Plasmids belonging to the IncQ family have an exceptionally broad host-range and are highlymobilizable in the presence of the self-transmissible IncP plasmids. All IncQ plasmids identified todate have certain features in common. The feature that distinguishes them most from all otherplasmids is that they have a unique mechanism of replication. Their replicons consist of repA, repBand repC genes encoding a replicase, primase and DNA-binding proteins respectively. All IncQplasmids contain at least three 22-bp iterons (or 20-bp iterons with 2-bp spacers) that are identicalin sequence and to which the RepC DNA-binding protein binds. They replicate by means of a uniquestrand-displacement mechanism that is considered to place a limit on their size. Replicationproceeds by a partially single-stranded intermediate that is believed to result in an increasedlikelihood of structural instability with an increase in plasmid size. The most compact backbone ofIncQ plasmids is approximately 5.9-kb and the largest natural IncQ plasmid reported is 14.2-kb.Although the mobilization regions of IncQ plasmids are not as unique as the replicons, they are allcharacterized by the primase of the replicon being fused to the relaxase of the mobilization genes.The remainder of the mobilization genes may vary substantially in number and sequence betweenplasmids and have been subdivided into at least four distinct lineages.This dissertation consists of twenty one manuscripts published during the period 1984 to 2012. Thefocus is almost entirely on the IncQ plasmid subfamily known as IncQ2. Most of the earlier work wason determining the nature and extent of the replicons, mobilization genes and the toxin-antitoxinplasmid stability system. A strong theme in the latter work focussed on using the natural variationamong the IncQ2 plasmids as a means to understand IncQ plasmid evolution. The collection ofarticles comprises a combination of original research and reviews.

[发布机构] Stellenbosch University