The transition from conjugal development to the first vegetative cell division is dependent on RAD51 expression in the ciliate Tetrahymena thermophila

Rad51p, the eukaryotic homolog of the prokaryotic recA protein. catalyzes s trand exchange between single- and double-stranded DNA and is involved in b oth genetic recombination and double-stranded break repair in the ciliate T etrahymena thermophila. We have previously showed that disruption of the Te trahymena RAD51 somatic macronuclear locus leads to defective germline micr onuclear division and that conjugation of two somatic rad51 null strains re sults in an early meiotic arrest. We have constructed Tetrahymena strains t hat are capable of RAD51 expression from their parental macronuclei and are homozygous, rad51 nulls in their germline micronuclei. These rad51 null he terokaryons complete all of the early and middle stages of conjugation, inc luding meiosis, haploid nuclear exchanger zygotic fusion, and the programme d chromosome fragmentations, sequence eliminations, and rDNA amplification that occur during macronuclear development. However, the rad51 null progeny fail to initiate the first vegetative cell division following conjugal dev elopment. Coincident with the developmental arrest is a disproportionate am plification of rDNA, despite the maintenance of normal total DNA content in the developing macronuclei. Fusion of arrested rad51 null exconjugants to wild-type cells is sufficient to overcome the arrest. Cells rescued by cyto plasmic fusion continue to divide, eventually recapitulating the micronucle ar mitotic defects described previously for rad51 somatic nulls.