λ lac :: cat variants A. Recombination between a 3.6 kbp linear dsDNA fragment and lac genes in the bacterial chromosome. A non-functional lacZ::cat allele is generated by the recombination event pictured. The linear DNA is released from the chromosome of a non-replicating λ phage, by the action of PaeR7 restriction endonuclease in the infected cell. The lac genes are shown as oriented in the conventional E. coli map. Their transcription is from right to left; replication forks travel through them from left to right. B. Structures of the cat substitutions in the λ phages used in this study. White bars represent cat and adjacent sequences from Tn9 (1 kbp). Colored bars represent lac sequences (1.3 kbp or 40 bp). PaeR7 sites are represented by P. All the substitutions replace the same λ sequences, bp 23,135–33,498, with the indicated sequences and an additional 1.9 kbp of sequence from phage P22 gene 9 (not shown in the diagrams). λ sequences replaced by the substitutions include the attachment site, int, xis, exo, bet, gam, and cIII. The substitutions result in net deletions of 5.0–7.6 kbp from the λ chromosome.