Integron-Containing IncU R Plasmids pRAS1 and pAr-32 from the Fish Pathogen Aeromonas salmonicida

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Abstract
A 45-kb R plasmid, pRAS1, that confers resistance to tetracyclines, trimethoprim, and sulfonamides was isolated in 1989 from an atypical strain of the fish pathogen Aeromonas salmonicida . This plasmid could be transferred by conjugation to Escherichia coli with a high degree of efficiency (frequency, 0.48). The following year pRAS1 was isolated from A. salmonicida subsp. salmonicida in the same area. Incompatibility group U plasmid pRAS1 contained a drug resistance-determining region of 12 kb consisting of a class 1 integron similar to In4 of Tn 1696 but with a dfrA16 gene cassette inserted. Close to IS 6100 at the right end of Tn4 was a truncated Tn 1721 . Restriction enzyme analysis showed that R plasmid pAr-32, isolated from A. salmonicida in Japan in 1970, had the same backbone structure as pRAS1, while the drug resistance-determining region contained a complex class 1 integron with an aadA2 cassette; the chloramphenicol resistance gene catA2 , as in In6 of pSa; and a duplicate of the 3′ conserved segment of the integron.