In the fission yeast Schizosaccharomyces pombe, 14 prp (pre-mRNAprocessing) mutants have been isolated to date. We cloned the prp10(+) gene by complementation of the temperature-sensitive growth of prp10. Five types of transcripts were found that were alternatively spliced with respect to two possible introns located in the 5'-terminal region. Three of them are probably functional and code for putative proteins of approximately 1200 amino acids. Proteins highly homologous to Prp10p are present in other organisms, one of which is a human spliceosome-associated protein SAP155, a subunit of the splicing factor complex SF3. The C-terminal two-thirds of Prp10p is highly conserved among species, and contains consensus repeats for the regulatory subunit A of protein phosphatase PP2A. A gene disruption experiment indicated that the prp10(+) gene is essential for viability in S.pombe. Prp10p tagged with GFP is predominantly localized in the nuclear DNA region. A series of deletions showed that the less conserved N-terminal region of approximately 300 amino acids in Prp10p is dispensable, although the corresponding region was thought to play important roles in the mammalian splicing system.