MUTATION OF THE SPS1-ENCODED PROTEIN-KINASE OF SACCHAROMYCES-CEREVISIAE LEADS TO DEFECTS IN TRANSCRIPTION AND MORPHOLOGY DURING SPORE FORMATION

During sporulation of Saccharomyces cerevisiae, meiosis is followed by encapsulation of haploid nuclei within multilayered spore walls. Comp letion of the late events of the sporulation program requires the SPS1 gene. This developmentally regulated gene, which is expressed as cell s are nearing the end of meiosis, encodes a protein with homology to s erine/threonine protein kinases. The catalytic domain of Sps1 is 44% i dentical to the kinase domain of yeast Ste20, a protein involved in th e pheromone-induced signal transduction pathway. Cells of a MATa/MAT a lpha sps1/sps1 strain arrest after meiosis and fail to activate genes that are normally expressed at a late time of sporulation. The mutant cells do not form refractile spores as assessed by phase-contrast micr oscopy and do not display the natural fluorescence and ether resistanc e that is characteristic of mature spores. Examination by electron mic roscopy reveals, however, that prospore-like compartments form in some of the mutant cells. These immature spores lack the cross-linked surf ace layer that surrounds wild-type spores and are more variable in siz e and number than are the spores of wild-type cells. Despite their ina bility to complete spore formation, sps1-arrested cells are able to re sume mitotic growth on transfer to rich medium, generating haploid pro geny. Our results suggest that the developmentally regulated Sps1 kina se is required for normal progression of transcriptional, biochemical, and morphological events during the later portion of the sporulation program.