The topology of homologous pairing : (genetic recombination, RecA protein, strand crossover, joint molecules, heteroduplex joints, branch migration)
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The RecA protein, which is essential for genetic recombination of E. coli , promotes homologous pairing in vitro. Studies of purified RecA protein have shown that it promotes the homologous pairing of a number of topological variants of DNA including : a) complementary linear single strands, b) linear single strands and duplex DNA, c) circular single strands and either linear duplex DNA, or nicked circular duplex DNA, and d) duplex DNA with a gap in one strand and closed circular duplex DNA. According to these observations RecA protein will form stable joint molecules if one molecule is single-stranded or partially single-stranded and if either molecule has a free end. Circular single strands, as well as linear ones, also stimulate RecA protein to unwind duplex DNA. These observations, which reveal a lack of specificity with regard to ends, suggest that RecA protein promotes homologous pairing of strands in a side-by-side fashion, and that free ends subsequently serve to stabilize the product by permitting the formation of heteroduplex regions with the normal right-handed helical structure.