Abstract
Several naturally occurring antibiotic resistance plasmids were isolated from Pasteurella multocida type D strains. One plasmid, pPM1, was used to study transfer of DNA among P. multocida strains, and could be transferred into Escherichia coli and some P. multocida isolates. However, pPM1 could only be transferred into the toxigenic P. multocida LFB3 at very low frequency. Plasmid recovered from the electrotransformants could be transferred to LFB3 at high frequency. These plasmid DNAs were resistant to PstI, and sensitive to DpnI digestion. Sensitivity to DpnI was common to all the P. multocida DNAs, but resistance to PstI was confined to LFB3. Plasmid pPM1 treated with PstI methylase was able to transform LFB3 at an increased frequency compared to unmethylated DNA, suggesting that LFB3 has a restriction system which cleaves at or near PstI sites.
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