Restriction-modification systems in Helicobacter pylori strains - from the evolutionary point of view

Restriction-modification (R-M) systems are believed to be the defense systems for bacteria by restricting foreign DNA and modifying their own DNA. More than three thousand restriction enzymes have been characterized and over two hundred specificities have been reported. Very little is known regarding the evolution of R-M systems and the generation of new specificities. Helicobacter pylori is a gram-negative bacterial pathogen implicated in a wide range of gastroduodenal diseases. More than fifteen putative type II restriction-modification (R-M) systems have been identified in the two completely sequenced H. pylori strains, 26695 and J99. Based on sequence comparison, almost 90% of the R-M genes are shared in these two strains. However, each strain displays unique R-M activities. Studies from other groups have shown extremely diverse R-M activities in different clinical isolates. Data from these studies provides an opportunity to study the biological function as well as the evolution of R-M systems. We have found that H. pylori maintains a dynamic and diversified set of R-M systems through mutation, recombination, and horizontal gene acquisition.