Genetic organization and evolution of the cryptic plasmid of Neisseria gonorrhoeae.

Three promoters have been identified on the phenotypically cryptic plasmid of Neisseria gonorrhoeae that can direct transcription in Escherichia coli. These promoters do not seem to function at a detectable level when N. gonorrhoeae is grown in vitro under normal growth conditions. The T7 RNA polymerase expression system has been used to identify the proteins encoded by this plasmid, and a series of subclones were constructed and used to localize the gene encoding each protein. In a search of the sequence databases, we have discovered that the derived amino acid sequence of a 1.2-kb segment of the plasmid shows a high degree of sequence similarity to the Mob proteins encoded by the colicinogenic plasmids ColA, ColE1, and ColK of E. coli. The 1.2-kb segment contains part of a Col mobilization region that has subsequently been inactivated by a small duplication. During the recombination event that formed the cryptic plasmid one of the mobilization genes was fused to a cryptic plasmid open reading frame to form the cppB gene. Despite the manner in which it was formed, we have shown that the cppB gene can be expressed when N. gonorrhoeae is grown under the appropriate environmental conditions.