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.