| Summary: | Thesis (DSc)--Stellenbosch University, 2014. === ENGLISH ABSTRACT: Plasmids belonging to the IncQ family have an exceptionally broad host-range and are highly
mobilizable in the presence of the self-transmissible IncP plasmids. All IncQ plasmids identified to
date have certain features in common. The feature that distinguishes them most from all other
plasmids is that they have a unique mechanism of replication. Their replicons consist of repA, repB
and repC genes encoding a replicase, primase and DNA-binding proteins respectively. All IncQ
plasmids contain at least three 22-bp iterons (or 20-bp iterons with 2-bp spacers) that are identical
in sequence and to which the RepC DNA-binding protein binds. They replicate by means of a unique
strand-displacement mechanism that is considered to place a limit on their size. Replication
proceeds by a partially single-stranded intermediate that is believed to result in an increased
likelihood of structural instability with an increase in plasmid size. The most compact backbone of
IncQ 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 all
characterized 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 between
plasmids and have been subdivided into at least four distinct lineages.
This dissertation consists of twenty one manuscripts published during the period 1984 to 2012. The
focus is almost entirely on the IncQ plasmid subfamily known as IncQ2. Most of the earlier work was
on determining the nature and extent of the replicons, mobilization genes and the toxin-antitoxin
plasmid stability system. A strong theme in the latter work focussed on using the natural variation
among the IncQ2 plasmids as a means to understand IncQ plasmid evolution. The collection of
articles comprises a combination of original research and reviews. === AFRIKAANSE OPSOMMING: Plasmiede wat aan die IncQ familie behoort kom ‘n uitsonderlike wye gasheerselreeks voor en is
hoogs mobiliseerbaar deur middel van die selfoordraagbaar IncP plasmiede. Alle IncQ plasmiedes
wat tot datum identifiseer is het sekere gemeenskaplike eienskappe. Die eienskap wat hulle van alle
ander plasmiedes onderskei is hul unieke dupliseringsmeganisme. Hul dupliseringsmeganisme
bestaan uit repA, repB en repC gene wat onderskeidlik ‘n helikase, ‘n ‘primase’ en ‘n DNSbindingsproteïen
enkodeer. Die IncQ plasmiede het ten minste drie 22-bp iterone (of 20-bp iterone
met 2-bp skeidingsnukleotiede) met ‘n identiese nukleotiedvolgorde en waaraan die RepCbindingsproteïen
bind. Hulle dupliseer deur middel van ‘n unieke DNA-string-vervangingsmeganisme
wat ‘n beperking op hul grootte plaas. Tydens replikasie word ‘n intermediêre struktuur gevorm wat
gedeeltelik enkelstring is en dit is blykbaar die rede vir ‘n verhoging in strukturële onstabilitiet as die
plasmied groter word. Die kleinste ruggraat onder die IncQ plasmiede is min of meer 5.9-kb en die
grootste natuurlike IncQ plasmied wat gerapporteer is, is 14.2-kb.
Alhoewel die mobiliseringsgebied van die IncQ plasmiede nie so duidelik uitkenbaar as die replikons
nie, hierdie gebied is gekenmerk deur ‘n ‘primase’ wat aan ‘n ‘relaxase’ in die mobiliseringsgene
gekoppel is. Die oorblywende mobiliseringsgene verskil in beide getal en nukleotiedvolgorde tussen
plasmiede en is gebruik om die plasmiede in vier duidelike oorsponggroepe in te deel.
Hierdie proefskrif bestaan uit een-en-twintig artikels wat tussen 1984 en 2012 gepubliseer is. Die
fokus is hoofsaaklik op die IncQ plasmiedsubfamilie wat as IncQ2 bekend is. Baie van die vroeër
werk het oor die aard en omvang van die duplisering en mobiliseringsgene asook die toksienteentoksien
plasmiedstabiliseringsmeganisme hanteer. ‘n Sterk tema in die latere werk was om die
natuurlike variasie onder die IncQ2 plasmiede te bestudeer ten einde IncQ plasmiedevolusie te
verstaan. Die publikasie versameling bestaan uit ‘n kombinasie van oorspronklike navorsing en
oorsigartikels.
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