Two polynucleotide-dependent ATPases, 95 and 181 kDa in size, have been purified to near homogeneity from cell-free extracts of Schizosaccharomyces pombe. Despite their size differences, their biochemical properties were strikingly similar. Both enzymes were capable of unwinding RNA and DNA duplexes in keeping with their ability to hydrolyze ATP in the presence of either ribo- or deoxyribopolynucleotide, In addition, they were capable of unwinding DNA/RNA or RNA/DNA hybrid duplexes and translocated in the 5' to 3' direction. These results strongly indicate that they are closely related to each other. Determination of the partial amino acid sequence of the 95-kDa enzyme revealed that it is encoded by the sen1(+) gene, an S. pombe homologue of yeast SEN1, a protein essential for the processing of small nucleolar RNA, transfer RNA, and ribosomal RNA. The molecular weight of the S. pombe Sen1 protein (SpSen1p) predicted from the sen1(+) open reading frame was 192.5 kDa, suggesting that the 181-kDa enzyme is likely to be a full-length protein, whereas the 95-kDa polypeptide has arisen by proteolysis. In accord with this possibility, polyclonal antibodies specific to the C-terminal region of sen1(+) cross-reacted with both 95- and 181-kDa polypeptides. We discuss the biochemical activities associated with SpSen1p and their relevance to the apparently divergent functions ascribed to the yeast Sen1 protein in RNA metabolism.