Summary: Bacteriophage Mu pf7701, a Km derivative of Mu 62, was inserted into the conjugative plasmid pDC251, which carries Tn. When pDC251 : : Mu pf7701 plasmids were conjugated from into derivatives of strain SS104 they were unstable; loss of either or both drug resistance markers occurred. Stable transconjugants resulted from deletion of Mu sequences, integration of the plasmid into the chromosome, or loss of an indigenous 34 kb cryptic plasmid. Among transconjugants selected for Km, the largest colonies arose from transconjugants in which Mu pf7701 had transposed to the chromosome and the pDC251 : : Mu pf7701 plasmid had been lost; 1300 transconjugants of this type were screened for pathogenicity to corn () seedlings and eight mutants were obtained that did not cause watersoaking symptoms. The insertions of Mu pf7701 in these mutants were in the chromosome.


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