“…To identify novel factors involved in pre-mRNA splicing, we and others have isolated prp (pre-mRNA processing) mutants of fission yeast Schizosaccharomyces pombe, which are defective in pre-mRNA splicing at the nonpermissive temperature+ So far, 14 prp mutants (prp1-prp14 ) have been isolated in S. pombe (Potashkin & Frendewey, 1989;Rosenberg et al+, 1991;Urushiyama et al+, 1996;Potashkin et al+, 1998)+ At the nonpermissive temperature, these mutants are defective in pre-mRNA splicing and accumulate pre-mRNAs in cells+ Interestingly, cell bodies in 8 of 14 prp mutants are elongated at the nonpermissive temperature, a typical phenotype observed in cell division cycle (cdc) mutants (Urushiyama et al+, 1996;Potashkin et al+, 1998)+ Expression of the cdc-like phenotype at the nonpermissive temperature in these mutants indicates a possible relation between pre-mRNA splicing and cell-cycle progression+ The prp1 ϩ gene was found to encode a protein with a TPR-motif (Urushiyama et al+, 1997) and to be allelic with zer1 (zero1), which was independently identified as a G0 arrest mutation (K+ Okazaki & H+ Okayama, unpubl+ results)+ The prp2 ϩ gene encodes spU2AF 59 , a fission yeast homolog of the human U2AF large subunit U2AF 65 (Potashkin et al+, 1993;Wentz-Hunter & Potashkin, 1996)+ The prp4 ϩ gene product is predicted to be a serine/threonine kinase (Alahari et al+, 1993;Groß et al+, 1997)+ The prp10 ϩ gene encodes a fission yeast homolog of SAP155/SF3b155, a subunit of the splicing factor complex SF3 associated with U2 snRNP (Habara et al+, 1998)+ Mutants of these four genes do not exhibit the cdc-like phenotype at the nonpermissive temperature+ On the other hand, two of the genes responsible for prp mutations with the cdc phenotype (prp5 and prp8) were cloned+ The prp5 ϩ gene encodes a WD repeat protein (McDonald et al+, 1999)+ Physical and genetic interactions between Prp5p and Cdc5p were also reported (McDonald et al+, 1999)+ The prp8 ϩ gene, the mutant of which exhibits the typical cdc phenotype, was found to be identical with the cdc28 ϩ gene encoding a DEAH-box RNA helicase (Lundgren et al+, 1996)+ However, exact mechanisms whereby defects in pre-mRNA splicing cause cell-cycle blocks in these mutants are still unknown+ SF3 is a U2 snRNP-associated multisubunit complex required for binding of U2 snRNA to the branch site+ It consists of two subcomplexes, SF3a and SF3b (reviewed in Krämer, 1996;Burge et al+, 1999)+ SF3a and SF3b contain three (SAP61/SF3a60, SAP62/SF3a66, and SAP114/SF3a120) and four subunits (SAP49/ SF3b50, SAP130/SF3b130, SAP145/SF3b145, and SAP155/SF3b155), respectively (Krämer et al+, 1994;…”