Methylation-dependent DNA discrimination in natural transformation ofCampylobacter jejuni

Campylobacter jejuni, a leading cause of bacterial gastroenteritis, is naturally competent. Like many competent organisms,C. jejunirestricts the DNA that can be used for transformation to minimize undesirable changes in the chromosome. AlthoughC. jejunican be transformed byC. jejuni-derived DNA, it is poorly transformed by the same DNA propagated inEscherichia colior produced with PCR. Our work indicates that methylation plays an important role in marking DNA for transformation. We have identified a highly conserved DNA methyltransferase, which we termCampylobactertransformation system methyltransferase (ctsM), which methylates an overrepresented 6-bp sequence in the chromosome. DNA derived from actsMmutant transformsC. jejunisignificantly less well than DNA derived fromctsM+(parental) cells. ThectsMmutation itself does not affect transformation efficiency when parental DNA is used, suggesting that CtsM is important for marking transforming DNA, but not for transformation itself. The mutant has no growth defect, arguing against ongoing restriction of its own DNA. We further show thatE. coliplasmid and PCR-derived DNA can efficiently transformC. jejuniwhen only a subset of the CtsM sites are methylated in vitro. A single methylation event 1 kb upstream of the DNA involved in homologous recombination is sufficient to transformC. jejuni, whereas otherwise identical unmethylated DNA is not. Methylation influences DNA uptake, with a slight effect also seen on DNA binding. This mechanism of DNA discrimination inC. jejuniis distinct from the DNA discrimination described in other competent bacteria.