The presence of DNA sequences Related to a 4.2Kb cryptic plasmid in the chromosomes of plasmid-free strains of Neisseria gonorrhoeae

Most strains of Neisseria gonorrhoeae contain a 4.2-Kb cryptic plasmid. However, strains that do not possess the 4.2Kb plasmid have been isolated. We examined 38 such strains and found that five of them contained chromosomal sequences homologous to the 4.2Kb plasmid. Restriction analysis and hybridization of 4.2Kb plasmid DNA to blots of the agarose gels suggested that homology with the 4.2Kb plasmid occurred at a single chromosomal site that was the same for all five strains. Similar experiments showed that these sequences derived from the 2.7Kb Hinfl (HinfIA) fragment of the cryptic plasmid. None of these five strains or two additional strains that exhibited no homology with the plasmid had sequences homologous with the 1.5Kb Hinfl fragment. When the region homologous to the HinfIA fragment was cloned from the chromosomal DNA of one strain, it was found to comprise the entire HinfIA fragment of the plasmid.