A DNase encoded by integrated element CJIE1 inhibits natural transformation of Campylobacter jejuni

The species Campylobacter jejuni (C. jejuni) is considered naturally competent for DNA uptake and displays strong genetic diversity. Nevertheless, non-naturally transformable strains and several relatively stable clonal lineages exist. In the present study, the molecular mechanism responsible for the non-natural transformability of a subset of C. jejuni strains was investigated. Comparative genome hybridization indicated that the C. jejuni Mu-like prophage integrated element 1 (CJIE1) was more abundant in non-naturally transformable C. jejuni strains than in naturally transformable strains. Analysis of CJIE1 indicated the presence of dns (CJE0256), which is annotated as an extracellular deoxyribonuclease. DNase assays using a defined dns mutant and a dns- strain expressing Dns from a plasmid indicated Dns as an endogenous DNase. The DNA hydrolyzing activity directly correlated with the natural transformability of the knockout mutant and the dns- strain expressing Dns from a plasmid. Analysis of a broader set of strains indicated that the majority of non-naturally transformable strains expressed DNase activity, while all naturally competent strains lacked this activity. The inhibition of natural transformation in C. jejuni via endogenous DNase activity possibly contributes to the formation of stable lineages in the C. jejuni population