SUMMARY: Chromosomal DNA from the Nine Mile phase I strain of (CB9MIC7) was cloned into the cosmid vector pHC79. The resulting gene library was probed with a radiolabelled III fragment present in the parental strain but absent from a spontaneously derived Nine Mile phase II strain (CB9MIIC4). The insert, which includes the missing III fragment, was 38.5 kb in length. When DNA from this cosmid clone was hybridized to genomic DNA of the parental CB9MIC7 and its derivative CB9MIIC4, a number of fragments were missing or altered in the latter strain. Restriction mapping localized the fragments to a contiguous portion of the chromosomal DNA fragment. The data were consistent with an 18 kb deletion in the chromosome of CB9MIIC4. Another intrastrain spontaneous derivative, CB9MI514, also lacked the sentinel III fragment and carried a deletion of approximately 29 kb within the same cloned insert. Both deletions appeared to share a common terminus, within the limits of resolution. In all other strains investigated, both phase I and phase II, the DNA represented by the insert seemed intact. The strains examined were representative of various stages of phase variation. The relationship between the observed deletions and the mechanism of phase transition in Nine Mile strains is discussed.


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