Directed Formation of Deletions and Duplications Using Mu d(Ap, lac)

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A genetic procedure is directed for the isolation of chromosomal deletions and duplications with predetermined endpoints. These rearrangements are generated in transduction crosses using a mixture of P22-transducing phage lysates grown on two strains, each carrying a Mud-lac insertion. The formation of duplications and deletions was demonstrated in the his operon using insertions of Mud1-8 (a transposition-defectiveMu d-lac phage). This technique was also used to make larger chromosomal duplications between Mud1-8 insertions in the thr and leu biosynthetic operons and between Mud insertions in the thr and pyrB operons. Genetic evidence is presented that strongly suggests that inheritance of a single Mud prophage by P22-mediated crosses requires two transduced fragments; each carrying part of the Mu d prophage. The two fragments must be involved in three recombinational exchanges; one exchange joins the donor Mud fragments and two exchanges occur between the composite fragment and the recipient chromosome, one on either side of the complete donor Mud element. Since duplications only occur between Mud insertions in the same orientation on the chromosome, the method of duplication formation provides a simple means of determining the orientation of Mud1-8 on the chromosome and, therefore, the direction of transcription of the gene into which Mud is inserted. This method was also used to construct recombinants between a Mud1-8 prophage and Casadaban's protein fusion vector Mud2 and, thereby, isolate Mud2-8, a Mud derivative containing the protein fusion ability of Mud2 and the defective transposition functions of Mud1-8.

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