Developmental Changes inTranslatable RNA Species Associated withMeiosis andSporeFormation inSaccharomyces cerevisiae

changes were duetothedenovo appearanceoftranslatable species, andtwoof these becamepredominant species ofthetotal population. Themajority ofthespecific changes weredueto either permanent ortransient increases intheconcentration ofindividual mRNA species; 11decreases were found. Changes were found atmoststages ofsporulation, although many occurred ineither oftwostages, oneearly (before 2h)andtheother later (between 6and8h)whencommitment tomeiotic segregation was beginning. Theresults provide evidence forbothquantitative and,toa lesser extent, qualitative transcriptional control ofgeneexpression during sporulation. Sporulation intheyeast Saccharomyces cerevisiae isa useful system forstudying development forseveral reasons: thegenetics ofS.cerei'isiae hasbeenextensively studied, andmanymutants withknownphysiological andbiochemicalcharacteristics areavailable; sporulation isreadily initiated,andrelatively synchronous populations ofcells invariousstages ofsporulation canbeobtained; onlyheterozygous MATa/MATadiploid cells cancomplete ascusformation undersporulation conditions, thehaploid andhomozygous diploids being unable tosporulate; andunderthenitrogen starvation conditions utilized fortheinduction ofsynchronoussporulation, mitotic cell division doesnotoccur, even incells thatdonotsporulate. Therefore, this development process isnotcomplicated byconcomitant cell cycle events. Thisalsoallows onetoestablish whichevents during this regimen areduesolely tostep-down bylooking atthe behavior ofnonsporulating strains. Tounderstand howgeneexpression isregulated during development, itisfirst necessary toidentify biochemical changes specifically associated withtheprocess. Genetic analyses haveimplicated atleast 50loci tocodeforfunctions indispensible totheprocess ofmeiosis andsporulation (16), andithasbeenestablished that sporulation inS.cerevisiae is dependent uponcertain vegetative geneproducts (29, 30). However, onlyaveryfewbiochemical events whichare unique tosporulation haveasyetbeenidentified (8,9,13, 38). Protein synthesis isessential formeiosis andsporulation since theaddition ofcycloheximide during culture under sporulation conditions resulted ininhibition ofthis developmental process (14, 25). Although thetotal protein content