Abstract
Helicobacter pylori chronically colonises half of the world’s human population and is the main cause of ulcers and gastric cancers. Its prevalence and the increase in antibiotic resistance observed recently reflect the high genetic adaptability of this pathogen. Together with high mutation rates and an efficient DNA recombination system, horizontal gene transfer through natural competence makes of H. pylori one of the most genetically diverse bacteria. We show here that transformation capacity is enhanced in strains defective for recN, extending previous work with other homologous recombination genes. However, inactivation of either mutY or polA has no effect on DNA transformation, suggesting that natural competence can be boosted in H. pylori by the persistence of DNA breaks but not by enhanced mutagenesis. The transformation efficiency of the different DNA repair impaired strains correlates with the number of transforming DNA foci formed on the cell surface and with the expression of comB8 and comB10 competence genes. Overexpression of the comB6-B10 operon is sufficient to increase the transformation capacity of a wild type strain, indicating that the ComB complex, present in the bacterial wall and essential for DNA uptake, can be a limiting factor for transformation efficiency.
Highlights
The expression of competence genes in H. pylori, among which several comB, was found to be elevated in addA mutants as well as after treatment with ciprofloxacin, an antibiotic that inhibits several DNA processing enzymes and generates DNA breaks[27]. These results suggested that the persistence of DNA strand breaks in the bacterial chromosome triggers an increase in transformation capacity
With the hypothesis that persistence of strand breaks can induce transformation, disruption of recN, another gene involved in strand break repair[33], resulted in a significant increase of recombinant frequencies when bacteria were transformed with genomic DNA from a streptomycin resistant strain (Table 1 and Fig. 1A)
To test whether the mutation burden affected transformation capacity, as proposed for S. pneumoniae[31], we used as recipients strains defective in DNA repair pathways affecting mutation frequencies
Summary
Computational and genetic analysis revealed that the membrane complex is encoded in H. pylori by two operons, comB2-B4 and comB6-B10, named after the A. tumefaciens orthologues[16,17]. This complex is essential for the entrance of transforming DNA (tDNA) into the periplasm. In the case of S. pneumoniae, the mutational burden in coding sequences caused by inactivation of a DNA repair pathway induces competence[31]. We show that transformation capacity is induced in strains mutated in HR genes such as addA, recO or recN, but not in the polA and mutY genes involved in other DNA repair pathways. Transformation efficiency correlates with DNA uptake capacity reflected in the formation of tDNA foci on the bacteria and, with the comB6-B10 operon expression levels
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