RAD51 is required for propagation of the germinal nucleus in Tetrahymena thermophila

RAD51, the eukaryote homolog of the Escherichia coli recA recombinase, part icipates in homologous recombination during mitosis, meiosis, and in the re pair of double-stranded DNA breaks. The Tetrahymena thermophila RAD51 gene was recently cloned, and the in vitro activities and induction of Rad51p fo llowing DNA damage were shown to be similar to that of RAD51 from other spe cies. This study describes the pattern of Tetrahymena RAD51 expression duri ng both the cell cycle and conjugation. Tetrahymena RAD51 mRNA abundance is elevated during macronuclear S phase during vegetative cell growth and wit h both meiotic prophase and new macronuclear development during conjugation . Gene disruption of the macronuclear RAD51 locus leads to severe abnormali ties during both vegetative growth and conjugation. rad51 nulls divide slow ly and incur rapid deterioration of their micronuclear chromosomes. Conjuga tion of two rad51 nulls leads to an arrest early during prezygotic developm ent (meiosis I). We discuss the potential usefulness of the ciliates' chara cteristic nuclear duality for further analyses of the potentially unique ro les of Tetrahymena RAD51.