Nuclear foci of mammalian recombination proteins are located at single-stranded DNA regions formed after DNA damage

A sensitive and rapid in situ method was developed to visualize sites of si ngle-stranded (ss) DNA in cultured cells and in experimental test animals. Anti-bromodeoxyuridine antibody recognizes the halogenated base analog inco rporated into chromosomal DNA only when substituted DNA is in the single st rand form. After treatment of cells with DNA-damaging agents or gamma irrad iation, ssDNA molecules form nuclear foci in a dose-dependent manner within 60 min. The mammalian recombination protein Rad51 and the replication prot ein A then accumulate at sites of ssDNA and form foci, suggesting that thes e are sites of recombinational DNA repair.