Construction of inversions. A. A plasmid containing the lac operon and some flanking sequences was constructed. The SphI site was converted to a BsrG1 site, the large BsrG1 lac segment was inverted, and the inverted operon was crossed into the bacterial chromosome. Sequence segments between certain boundaries – the inversion endpoints, the ends of the lac flanks, and the 12 bp of lacZ replaced by the cat gene – are given letter designations to simplify representations of parent and recombinant chromosomes in crosses described below. B. The structure of λ lac::cat930 is represented as a linear map. T designates the left side flanking λ sequences shared by plasmids and phages, consisting of λ bp 22346–23134. 9 designates phage P22 bp 17725–15955. R is the right side λ flank, bp 33502–34504. P is the rest of the phage λ chromosome. The lac::cat segment was inverted relative to the flanking sequences in plasmids pTP930 and 931 (generating pTP1032 and 1033), and the inverted elements were crossed into λ. The structure of λ lac::cat1033 is illustrated.