Diversity within the Campylobacter jejuni type I restriction–modification loci

<jats:p>The type I restriction–modification (<jats:italic>hsd</jats:italic>) systems of 73 <jats:italic>Campylobacter jejuni</jats:italic> strains were characterized according to their DNA and amino acid sequences, and/or gene organization. A number of new genes were identified which are not present in the sequenced strain NCTC 11168. The closely related organism <jats:italic>Helicobacter pylori</jats:italic> has three type I systems; however, no evidence was found that <jats:italic>C. jejuni</jats:italic> strains contain multiple type I systems, although <jats:italic>hsd</jats:italic> loci are present in at least two different chromosomal locations. Also, unlike <jats:italic>H. pylori</jats:italic>, intervening ORFs are present, in some strains, between <jats:italic>hsdR</jats:italic> and <jats:italic>hsdS</jats:italic> and between <jats:italic>hsdS</jats:italic> and <jats:italic>hsdM</jats:italic>. No definitive function can be ascribed to these ORFs, designated here as <jats:italic>rloA–H</jats:italic> (<jats:underline>R</jats:underline>-<jats:underline>l</jats:underline>inked <jats:underline>O</jats:underline>RF) and <jats:italic>mloA–B</jats:italic> (<jats:underline>M</jats:underline>-<jats:underline>l</jats:underline>inked <jats:underline>O</jats:underline>RF). Based on parsimony analysis of amino acid sequences to assess character relatedness, the <jats:italic>C. jejuni</jats:italic> type I R–M systems are assigned to one of three families: ‘IAB’, ‘IC’ or ‘IF’. This study confirms that HsdM proteins within a family are highly conserved but share little homology with HsdM proteins from other families. The ‘IC’ <jats:italic>hsd</jats:italic> loci are >99 % identical at the nucleotide level, as are the ‘IF’ <jats:italic>hsd</jats:italic> loci. Additionally, whereas the nucleotide sequences of the ‘IAB’ <jats:italic>hsdR</jats:italic> and <jats:italic>hsdM</jats:italic> genes show a high degree of similarity, the nucleotide sequences of the ‘IAB’ <jats:italic>hsdS</jats:italic> and <jats:italic>rlo</jats:italic> genes vary considerably. This diversity suggests that recombination between ‘IAB’ <jats:italic>hsd</jats:italic> loci would lead not only to new <jats:italic>hsdS</jats:italic> alleles but also to the exchange of <jats:italic>rlo</jats:italic> genes; five <jats:italic>C. jejuni hsd</jats:italic> loci are presumably the result of such recombination. The importance of these findings with regard to the evolution of <jats:italic>C. jejuni</jats:italic> type I R–M systems is discussed.</jats:p>