Site-specific cleavage of left-handed DNA in pBR322 by lambda-tris(diphenylphenanthroline)cobalt(III).

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Abstract
The chiral complex tris(4,7-diphenyl-1,10-phenanthroline)cobalt(III), .LAMBDA.-Co(DiP)33+, binds to and, with photoactivation, cleaves leaf-handed DNA helices, thereby providing a unique molecular probe for local DNA conformation. We have mapped the specific left-handed sites where .LAMBDA.-Co(DiP)33+ cleaves in the plasmids pBR322 and pLP32, which is the derivative of pBR322 containing a Z-form (C-G)16 insert. For pLP32, a primary cleavage is at the insert; for native pBR322, cleavage occurs at four discrete sites: 1.45, 2.3, 3.3, and 4.2 kilobase pairs. These sites correspond to segments of alternating purine-pyrimidines. Moreover, these positions map to the ends of the three distinct coding regions in pBR322; the tetracycline-resistance gene, the origin of replication, and either end of the ampicillin-resistance (.beta.-lactamase) gene. The locations of these left-handed segments suggest to us that Z-DNA might serve as a conformational punctuation mark to demarcate the ends of genes.