Haemophilus ducreyi strain ATCC 27722 contains a genetic element with homology to the vibrio RS1 element that can replicate as a plasmid and confer NAD independence on haemophilus influenzae.

The nucleotide sequence of pNAD1, a plasmid from Haemophilus ducreyi identified on the basis of its ability to confer NAD independence on Actinobacillus pleuropneumoniae and H. influenzae, has been determined. In addition to containing the nadV gene, the plasmid contains homologues of the rstR and rstA genes, genes encoding repressor and replication proteins, respectively, in the Vibrio CTXphi and the Vibrio RS1 element, suggesting a single-stranded bacteriophage origin for pNAD1. Tandem copies of the plasmid are integrated into the H. ducreyi 35000HP genome.