Origins of Gene Conversion and Reciprocal Exchange in Ascobolus

A heteroduplex joint containing paired strands from each of two DNA molecules has long been shown to mediate the faithful rejoining of pieces of DNA from different parents (Meselson 1967; Fox 1978). In vitro formation of long heteroduplex stretches has been studied extensively as a consequence of the action of the RecA protein of Escherichia coli (Dressler and Potter 1982; Radding 1982). A RecA-like protein has also been found in the lower eukaryote Ustilago (Kmiec and Holloman 1984). E. coli RecA protein can also promote both asymmetrie strand transfer and adjacent reciprocal strand exchange between two homologous circular duplexes, providing that one is nicked and the other bears a homologous gap (West et al. 1983). In this situation, the reciprocal strand exchange is limited by a Holliday junction that is blocked by a 400-bp deletion heterology. Such in vitro-built figure-eight dimers are resolved in vivo in both parental (the two...