The nucleotide excision repair (NER) system of Helicobacter pylori: role in mutation prevention and chromosomal import patterns after natural transformation.

BACKGROUND: Extensive genetic diversity and rapid allelic diversification are characteristics of the human gastric pathogen Helicobacter pylori, and are believed to contribute to its ability to cause chronic infections. Both a high mutation rate and frequent imports of short fragments of exogenous DNA during mixed infections play important roles in generating this allelic diversity. In this study, we used a genetic approach to investigate the roles of nucleotide excision repair (NER) pathway components in H. pylori mutation and recombination. RESULTS: Inactivation of any of the four uvr genes strongly increased the susceptibility of H. pylori to DNA damage by ultraviolet light. Inactivation of uvrA and uvrB significantly decreased mutation frequencies whereas only the uvrA deficient mutant exhibited a significant decrease of the recombination frequency after natural transformation. A uvrC mutant did not show significant changes in mutation or recombination rates; however, inactivation of uvrC promoted the incorporation of significantly longer fragments of donor DNA (2.2-fold increase) into the recipient chromosome. A deletion of uvrD induced a hyper-recombinational phenotype. CONCLUSIONS: Our data suggest that the NER system has multiple functions in the genetic diversification of H. pylori, by contributing to its high mutation rate, and by controlling the incorporation of imported DNA fragments after natural transformation.

The Helicobacter pylori mutY homologue HP0142 is an antimutator gene that prevents specific C to A transversions.

Extensive genetic variability resulting from a high mutation rate and frequent recombination is a characteristic of Helicobacter pylori. Its average mutation rate is 1 x 10(-6), similar to that of Escherichia coli mutator strains. Few genes involved in DNA repair have been functionally characterized in H. pylori. In E. coli, the DNA glycosylase MutY is a part of the base excision repair system. The H. pylori mutY homolog HP0142 was analyzed in this study. HP0142 was disrupted by inserting a kanamycin resistance cassette. Mutation rates were determined by measuring the frequency of point mutations in rpoB conferring resistance against rifampicin. Inactivation of mutY in H. pylori resulted in an increase of the mutation frequency by a factor of up to 34. Sequence analysis of rpoB in rifampicin-resistant clones selected from the mutY mutant showed a modest increase of G:C/T:A transversions in comparison to clones selected from wild type strains. In contrast, inactivation of mutY had a profound impact on the distribution of mutations within rpoB. This finding suggests that the efficiency with which mutY prevents transversions is strongly dependent upon the sequence context. Inactivation of mutY was associated with a stationary phase fitness deficit in competitive cultures with the wild type strain.

Mosaic DNA imports with interspersions of recipient sequence after natural transformation of Helicobacter pylori.

Helicobacter pylori colonizes the gastric mucosa of half of the human population, causing gastritis, ulcers, and cancer. H. pylori is naturally competent for transformation by exogenous DNA, and recombination during mixed infections of one stomach with multiple H. pylori strains generates extensive allelic diversity. We developed an in vitro transformation protocol to study genomic imports after natural transformation of H. pylori. The mean length of imported fragments was dependent on the combination of donor and recipient strain and varied between 1294 bp and 3853 bp. In about 10% of recombinant clones, the imported fragments of donor DNA were interrupted by short interspersed sequences of the recipient (ISR) with a mean length of 82 bp. 18 candidate genes were inactivated in order to identify genes involved in the control of import length and generation of ISR. Inactivation of the antimutator glycosylase MutY increased the length of imports, but did not have a significant effect on ISR frequency. Overexpression of mutY strongly increased the frequency of ISR, indicating that MutY, while not indispensable for ISR formation, is part of at least one ISR-generating pathway. The formation of ISR in H. pylori increases allelic diversity, and contributes to the uniquely low linkage disequilibrium characteristic of this pathogen.