The human recombination strand exchange process.

A mechanism for the initiation of general recombination that involves the formation of left-handed Z-DNA heteroduplex segments adjacent to right-handed B-DNA heteroduplex segments is discussed. The paranemic nature of this initiation structure allows for homology recognition in the absence of strand cleavage. This model suggests that proteins catalyzing recombination initiation via the formation of paranemic joint should in some capacity recognize Z-DNA. Other studies have shown that both the RecA protein of Escherichia coli and the Rec1 protein of Ustilago maydis have a greater affinity for Z-DNA than B-DNA. Here we have used Z-DNA affinity chromatography to purify a peptide of approximately 120 kilodaltons from a human tumor cell line that catalyzes a simple recombination strand-transfer reaction similar to one developed for the characterization of the RecA and Rec1 proteins. We report details of the characterization of the human strand-transfer activity and identified a potential human recombination complex.