Neisseria gonorrhoeae secretes chromosomal DNA via a novel type IV secretion system

The process of DNA donation for natural transformation of bacteria is poorly understood and has been assumed to involve bacterial cell death. Recently in Neisseria gonorrhoeae we found that mutations in three genes in the gonococcal genetic island (GGI) reduced the ability of a strain to act as a donor in transformation and to release DNA into the culture. To better characterize the GGI and the process of DNA donation, the 57 kb genetic island was cloned, sequenced and subjected to insertional mutagenesis. DNA sequencing revealed that the GGI has characteristics of a horizontally acquired genomic island and encodes homologues of type IV secretion system proteins. The GGI was found to be incorporated near the chromosomal replication terminus at the dif site, a sequence targeted by the site-specific recombinase XerCD. Using a plasmid carrying a small region of the GGI and the associated dif site, we demonstrated that this model island could be integrated at the dif site in strains not carrying the GGI and was spontaneously excised from that site. Also, we were able to delete the entire 57 kb region by transformation with DNA from a strain lacking the GGI. Thus the GGI was likely acquired and integrated into the gonococcal chromosome by site-specific recombination and may be lost by site-specific recombination or natural transformation. We made mutations in six putative type IV secretion system genes and assayed these strains for the ability to secrete DNA. Five of the mutations greatly reduced or completely eliminated DNA secretion. Our data indicate that N. gonorrhoeae secretes DNA via a specific process. Donated DNA may be used in natural transformation, contributing to antigenic variation and the spread of antibiotic resistance, and it may modulate the host immune response.