REGULATION AND INTRACELLULAR-LOCALIZATION OF SACCHAROMYCES-CEREVISIAESTRAND EXCHANGE PROTEIN-1 (SEP1 XRN1/KEM1), A MULTIFUNCTIONAL EXONUCLEASE/

The Saccharomyces cerevisiae strand exchange protein 1 (Sep1; also ref erred to as Xrn1, Kem1, Rar5, or Stp beta) catalyzes the formation of hybrid DNA from model substrates in vitro. The protein is also a 5'-to -3' exonuclease active on DNA and RNA. Multiple roles for the in vivo function of Sep1, ranging from DNA recombination and cytoskeleton to R NA turnover, have been proposed. We show that Sep1 is an abundant prot ein in vegetative S. cerevisiae cells, present at about 80,000 molecul es per diploid cell. Protein levels were not changed during the cell c ycle or in response to DNA-damaging agents but increased twofold durin g meiosis. Cell fractionation and indirect immunofluorescence studies indicated that >90% of Sep1 was cytoplasmic in vegetative cells, and i ndirect immunofluorescence indicated a cytoplasmic localization in mei otic cells as well. The localization supports the proposal that Sep1 h as a role in cytoplasmic RNA metabolism. Anti-Sep1 monoclonal antibodi es detected cross-reacting antigens in the fission yeast Schizosacchar omyces pombe, in Drosophila melanogaster embryos, in Xenopus laevis, a nd in a mouse pre-B-cell line.