ATP-independent DNA strand transfer catalyzed by protein(s) from meiotic cells of the yeast Saccharomyces cerevisiae.

An activity that catalyzes the transfer of a strand from a duplex linear molecule of DNA to a complementary circular single strand can be detected in crude extracts from mitotic and meiotic cells of the yeast Saccharomyces cerevisiae by adding yeast single-stranded DNA binding proteins. This DNA strand-transfer activity increases greater than 15-fold during meiosis in MATa/MAT alpha diploids prior to the detection of a 100- to 1000-fold increase in homologous chromosomal recombination. No increase is observed in MATa/MATa or MAT alpha/MAT alpha cells, which do not undergo meiosis when shifted to meiotic medium, suggesting the activity is related to meiotic recombination. The activity is named strand-transfer protein alpha (STP alpha) and has been extensively purified from the meiotic cells (6 hr after exposure to sporulation medium). The apparent molecular mass of STP alpha is 38 kDa under denaturing conditions. The DNA strand-transfer reaction catalyzed by STP alpha requires homologous single-stranded and double-stranded DNA and Mg2+ but no nucleotide cofactor. Yeast single-stranded DNA binding proteins stimulate the reaction at least 10-fold. Among the products analyzed by electron microscopy were typical strand-exchange structures.