The topology of homologous pairing : (genetic recombination, RecA protein, strand crossover, joint molecules, heteroduplex joints, branch migration)

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.CHARLES M. RADDING, CHANCHAL DAS GUPTA, TAKEHIKO SHIBATA and RICHARD P. CUNNINGHAM, Departments of Human Genetics and Molecular Biophysics and Biochemistry, Yale University School of Medicine, New Haven, CT