MEIOSIS-DEPENDENT MESSENGER-RNA SPLICING OF THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE MES1(+) GENE

The mesl(+) gene of the fission yeast Schizosaccharomyces pombe is ess ential for the second meiotic division. We have cloned a 1.1-kb HindII I fragment containing mesl(+) by complementation from an S. pombe geno mic library. Sequencing of the genomic and cDNA fragments indicates th e existence of one small intron of 75 nucleotides, although both the S '(G/GTTAGT) and 3'(CAG/T) intron-exon junctions deviate from the conse nsus sequences proposed for S. pombe. The putative translation product of the mature mesl(+) mRNA is a 11-kDa protein of 101 amino acids whi ch has no significant homology to any previously-reported proteins. Di sruption of mesl has no effect on cell growth but causes an arrest of meiosis before the second meiotic division. Northern-blot analysis rev ealed that mesl(+) was preferentially transcribed under conditions of nitrogen starvation. When a h(90) homothallic strain was shifted to a nitrogen-deficient medium, a pre-mRNA accumulated and then was gradual ly processed to generate a mature mRNA. This splicing did not occur in either a heterothallic haploid strain or in a homothallic mei2 mutant strain which was defective in tile initiation of meiosis. Expression of the first exon alone was not able to suppress the mes1 null allele. These results indicate that mesl(+) is required for the completion of meiosis, that splicing is required for the function of the mesl(+) ge ne, and that this splicing requires the function of the mei2(+) produc t.