SWM1, a Developmentally Regulated Gene, Is Required for Spore Wall Assembly in Saccharomyces cerevisiae

Meiosis inSaccharomyces cerevisiae is followed by encapsulation of haploid nuclei within multilayered spore walls. Formation of this spore-specific wall requires the coordinated activity of enzymes involved in the biosynthesis of its components. Completion of late events in the sporulation program, leading to spore wall formation, requires theSWM1 gene.SWM1 is expressed at low levels during vegetative growth but its transcription is strongly induced under sporulating conditions, with kinetics similar to those of middle sporulation-specific genes. Homozygousswm1 Δ diploids proceed normally through both meiotic divisions but fail to produce mature asci. Consistent with this finding,swm1 Δ mutant asci display enhanced sensitivity to enzymatic digestion and heat shock. Deletion ofSWM1 specifically affects the expression of mid-late and late sporulation-specific genes. All of the phenotypes observed are similar to those found for the deletion ofSPS1 orSMK1 , two putative components of a sporulation-specific MAP kinase cascade. However, epistasis analyses indicate that Swm1p does not form part of the Sps1p-Smk1p-MAP kinase pathway. We propose that Swm1p, a nuclear protein, would participate in a different signal transduction pathway that is also required for the coordination of the biochemical and morphological events occurring during the last phase of the sporulation program.