Nucleases Encoded by Integrated Elements CJIE2 and CJIE4 Inhibit Natural Transformation of Campylobacter jejuni

The species Campylobacter jejuni is naturally competent for DNA uptake; nevertheless, nonnaturally transformable strains do exist. For a subset of strains we previously showed that a periplasmic DNase, encoded by dns, inhibits natural transformation in C. jejuni. In the present study, genetic factors coding for DNase activity in absence of dns were identified. DNA arrays indicated that nonnaturally transformable dns-negative strains contain putative DNA/RNA non-specific endonucleases encoded by CJE0566 and CJE1441 of strain RM1221. These genes are located on C. jejuni integrated element 2 and 4. Expression of CJE0566 and CJE1441 from strain RM1221 and a homologous gene from strain 07479 in DNase-negative Escherichia coli and C. jejuni strains indicated that these genes code for DNases. Genetic transfer of the genes to a naturally transformable C. jejuni strain resulted in a decreased efficiency of natural transformation. Modelling suggests that the C. jejuni DNases belong to the Serratia nuclease family. Overall, the data indicate that the acquisition of prophage encoded DNA/RNA non-specific endonucleases inhibits the natural transformability of C. jejuni through hydrolysis of DNA. Overall design: The genomic diversity of 15 naturally competent or nonnaturally transformable Campylobacter jejuni strains were examined by microarray-based comparative genomic indexing (CGI) analysis. The CGI analysis allowed the assessment of CDS content for each C. jejuni strain relative to the C. jejuni DNA microarray, which comprises ORFs from strains NCTC 11168, RM1221. ORFs were spotted in duplicate. Genomic DNA from strains NCTC 11168/RM1221 were used as a reference DNA and competitively hybridized with genomic DNA from each of the other C. jejuni strains. Two replicates for each strain were performed. Data normalization was performed as in Parker et al. J Clin Microbiol 2006, 44(11):4125-4135.