Relationships among some R plasmids found in Haemophilus influenzae

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Abstract
Tetracycline resistance in a strain of H. influenzae isolated in the United Kingdom was determined by an apparently non-selftransmissible plasmid of 31 .times. 106 daltons (31 MDal), designated pUB701. DNA hybridization studies indicated that pUB701 shares about 70% base sequence homology with the 30-MDal ampicillin resistance R plasmid RSF007, isolated in the USA from H. influenzae, and 64% sequence homology with the 38-MDal tetracycline and chloramphenicol resistance R plasmid pRI234, isolated in the Netherlands. Heteroduplex studies between RSF007 and pUB701 confirmed the fact that these plasmids were largely homologous, except that pUB701 contained the tetracycline resistance transposon TnD, while RSF007 contained the ampicillin resistance transposon TnA. A strain of H. parainfluenzae resistant to chloramphenicol and tetracycline carried 2 species of plasmid DNA of 2.7 and 0.75 MDal. It was not proven that either resistance was plasmid-borne in this strain. Hybridization studies with a [3H]thymine-labeled tetracycline resistance enteric plasmid suggested that the tetracycline transposon was integrated into the chromosome of H. parainfluenzae UB2832. The strains studied apparently received R factors of the same incompatibility group bearing different resistance genes, or different resistance genes may have been translocated to a common resident plasmid of H. influenzae.