Molecular Mechanisms Influencing Genetic Diversity of Campylobacter jejuni

Abstract

Campylobacter jejuni is an important food-borne pathogen, causing human bacterial gastroenteritis. Throughout the years several methods have been developed for typing C. jejuni. These methods uncovered the existence of enormous genetic diversity within the species. Stable lineages of C. jejuni are also found, suggesting that some C. jejuni clones can survive without the need for changing their genome. Both horizontal gene transfer and intragenomic alterations contribute to the genetic diversity of C. jejuni. The aim of the research was to investigate molecular mechanisms that influence genetic diversity of C. jejuni. The subset and specificity of DNA repair mechanisms differ between bacterial species and the presence and functionality of three excision repair mechanisms (MMR, NER, and BER) and RecA-dependent recombinational repair in C. jejuni was investigated. The data indicated that C. jejuni has at least two functional DNA repair mechanisms, namely NER and recombinational repair. Further we attempted to unravel the basis of the clonal behavior of distinct C. jejuni strains using a comparative genomic approach. Systematic analysis of DNA microarrays probed with DNA of naturally transformable and nonnaturally transformable C. jejuni strains resulted in the identification of Dns. Dns is a periplasmic DNase, encoded by dns and located on C. jejuni integrated element 1. Dns inhibits natural transformation through hydrolysis of exogenous DNA. Since also nonnaturally transformable DNase+ C. jejuni without dns exist, an attempt to identify and functionally characterize additional DNase encoding gene(s) was undertaken. Comparative genomic hybridization for nonnaturally transformable DNase+ C. jejuni strains with or without dns indicated that a subset of dns- strains contain putative DNA/RNA non-specific endonucleases (CJE0566 and CJE1441). These genes are located on C. jejuni integrated element 2 and 4, respectively. It was shown that these genes code for DNases and that the gene products reduce the natural transformability of C. jejuni. By identification of three periplasmic nucleases a genetic basis for the lack of natural transformation in C. jejuni is provided. Finally, recombinant C. jejuni Dns was produced and its physical and catalytic properties were determined. Based on the results Dns can be classified as a sugar non-specific endonuclease that belongs to the family of beta-beta-alpha-metal topology containing nucleases. Due to its characteristic combination of functional properties Dns might be a valuable tool for specific industrial purposes. Overall, at least two functional DNA repair mechanisms were identified that may protect C. jejuni against DNA damage, which in doing so may limit the generation of genetic diversity. However, the apparent lack of a mechanism involved in repair of point mutations (MMR) may contribute to the diversity of C. jejuni, which may increase the adaptability of C. jejuni under stress conditions. Further, the molecular basis for the lack of natural transformability in a subset of C. jejuni strains was established. The responsible periplasmic nucleases may limit the spread of genes within the C. jejuni population, and can contribute to the formation of stable lineages. The fact that these nucleases are encoded by phage-like integration elements demonstrates that transduction can influence diversity in C. jejuni.