Paul T. Morrison

An activity that catalyzes the formation of joint molecules from linear M13mp19 replicative form DNA and circular M13mp19 viral DNA was purified 1000- to 2000-fold from mitotic Saccharomyces cerevisiae cells. The activity appeared to reside in a Mr 132,000 polypeptide. The reaction required that the substrates be homologous and also required Mg2+. There was no requirement for ATP. The reaction required stoichiometric amounts of protein and showed a cooperative dependence on protein concentration. Electron microscopic analysis of the joint molecules indicated they were formed by displacement of one strand of the linear duplex by the single-stranded circular molecule. This analysis also showed that heteroduplex formation started at the 3'-homologous end of the linear duplex strand followed by extension of the hybrid region toward the 5'-homologous end of the linear duplex strand (3'-to-5' direction).