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A putative DNA adenine methyltransferase is involved in Yersinia pseudotuberculosis pathogenicity

Yersinia Research Unit, Institut Pasteur, 28 rue du Dr Roux, 75724 Paris Cedex 15, France

Correspondence
Elisabeth Carniel
carniel2{at}pasteur.fr

Some adenine methyltransferases have been shown not only to protect specific DNA restriction sites from cleavage by a restriction endonuclease, but also to play a role in various bacterial processes and sometimes in bacterial virulence. This study focused on a type I restriction–modification system (designated yrmI) of Y. pseudotuberculosis. This system is composed of three adjacent genes which could potentially encode an N⁶-adenine DNA methylase (YamA), an enzyme involved in site-specific recognition (YrsA) and a restriction endonuclease (YreA). Screening of 85 isolates of Y. pestis and Y. pseudotuberculosis indicated that the yrmI system has been lost by Y. pestis and that yamA (but not yrsA or yreA) is present in all Y. pseudotuberculosis strains tested, suggesting that it may be important at some stages of the epidemiological cycle of this species. To further investigate the role of yamA in Y. pseudotuberculosis survival, multiplication or virulence, a yamA mutant of Y. pseudotuberculosis IP32953 was constructed by allelic exchange with a kanamycin cassette. The fact that yamA mutants were obtained indicated that this gene is not essential for Y. pseudotuberculosis viability. The IP32953yamA mutant strain grew as well as the wild-type in a rich medium at both 28 °C and 37 °C. It also grew normally in a chemically defined medium at 28 °C, but exhibited a growth defect at 37 °C. In contrast to the Dam adenine methyltransferase, a mutation in yamA did not impair the functions of DNA repair or resistance to detergents. However, the yamA mutant exhibited a virulence defect in a mouse model of intragastric infection. The in silico analysis indicated that the chromosomal region carrying the Y. pseudotuberculosis yrmI locus has been replaced in Y. pestis by a horizontally acquired region which potentially encodes another methyltransferase. YamA might thus be dispensable for Y. pestis growth and virulence because this species has acquired another gene fulfilling the same functions.

Two supplementary tables and a sequence alignment are available with the online version of this paper.

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