Molecular nature of a plasmid specifying beta-lactamase production in Haemophilus ducreyi

We characterized pJB1, the plasmid previously reported to mediate beta-lactamase production in Haemophilus ducreyi. We studied its relationship to pMR0360 and RSF0885, the plasmids responsible for beta-lactamase production in Neisseria gonorrhoeae and Haemophilus parainfluenzae, respectively. Although pJB1 was maintained as a multicopy pool in Escherichia coli, it was not stably maintained in the absence of antibiotic selection. Electron microscope heteroduplex studies showed that it carried 100% of the transposable ampicillin resistance sequence TnA. This sequence was transposed to plasmid pUB307 at a low rate. Heteroduplexes between pMR0360 and pJB1 showed that they contained 3.3 megadaltons of homologous sequences. Two sets of nonhomologous sequences, one a TnA sequence and the other a non-TnA sequence, took the form of insertion loops. For plasmids pMR0360 and RSF0885, previously shown to be highly related, the nonhomologous sequences took the form of a substitution loop. We concluded that all three plasmids shared major portions of their sequences but differed in discrete segments. pJB1 was the first such plasmid to have a physically and functionally intact TnA sequence.