An analysis of the complete nucleotide sequence of the Haemophilus ducreyi broad-host-range plasmid pLS88

We present an analysis of the complete nucleotide sequence of pLS88, a naturally occurring, 4.8-kb broad-host-range plasmid isolated from Haemophilus ducreyi and encoding resistance to sulfonamides, streptomycin, and kanamycin. Sequence analysis of the genes encoding sulfonamide and streptomycin resistance revealed homology to the RSF1010 sulII and strA genes. The sulII-strA intergenic region of pLS88 has a 38-bp deletion identical to that of the RSF1010-like plasmid pHD8.1, isolated from Actinobacillus pleuropneumoniae. The kanamycin resistance gene shows strong homology to Tn903, but lacks the inverted repeats of the transposon. No other genes have been identified. The region downstream of the kanamycin resistance gene shows homology to the RSF1010 oriV region; however, this region is not essential to plasmid replication. The ori of pLS88 is contained within a 1060-bp region and does not appear to contain structures typical of plasmid origins. This region is flanked by DNA showing strong homology to regions both upstream and downstream of the Haemophilus influenzae ROB-1 beta-lactamase gene. Because of the small size of the origin, pLS88 appears to resemble the structure of narrow-host-range plasmids, but replicates, via an as yet unidentified mechanism, as a broad-host-range plasmid.