Characterization ofYeastStrains withConditionally Expressed Variants ofRibosomal Protein Genestcmlandcyh2

We placed aregulatory sequence derived fromtheGALIOlocus ofSaccharomyces cerevisiae atvarious distances fromthestart sites oftranscription oftwoyeast ribosomal protein genes, tcmlandcyh2. Thehybrid ribosomal protein genes weretranscribed atwild-type levels inthepresence ofgalactose. Intheabsence of galactose, thehybrid genes weretranscribed either atareduced level oressentially notatall. Yeast cells which transcribe theribosomal protein genes ata reduced ratecontinued togrow,suggesting thatenhanced translation oftheribosomal protein mRNA maypermit anadequate rateofsynthesis ofthecorresponding protein. Consistent withthis suggestion isthefinding thatpreexisting mRNA decayed atareduced ratewhen transcription washalted abruptly byremoval ofgalactose. Yeast cells unable totranscribe termorcyh2without galactose didnotgrow. Theseconditional lethal strains demonstrate thattheribosomal proteins encoded by term andcyh2areessential; furthermore, these strains arepotentially useful forisolating mutations inthetcml andcyh2proteins affecting their transport, assembly, orfunction. Eucaryotic cells depend onthebiogenesis ofnumerous complex multimeric structures forgrowth. Histones and other nuclear proteins organize thegenomeandfacilitate its expression andreplication. Actins, tubulins, andavariety of other filamentous proteins polymerize toconstruct theinter- nalframework ofthecell andtoproduce formandmove- ment.Assembly ofover70different ribosomal proteins aboutfourrRNAmolecules gives rise totheprotein syn- thetic machinery ofthecell. Fromabiochemical standpoint suchcomplexes areinteresting systems inwhich tostudy the relationship between macromolecular assembly andfunc- tion. Moreover, there isampleevidence thatsynthesis of manyofthesehousekeeping proteins isinstrumental in regulating cell growth (18, 29). Thus, attention hasalso been focused uponidentifying themechanisms controlling the synthesis ofthese macromolecules. InSaccharomyces cerevisiae, asinother eucaryotes, each of70orsoribosomal proteins issynthesized atthesame relative rate. Immediately uponsynthesis theyaretrans- ported tothenucleolus, wheretheyassemble atarate which issufficient tomeettheribosome requirements ofaparticu- largrowth state yetwhichleaves nosignificant fraction of unassembled proteins. Inaddition, stimuli whichalter the growth ratearemanifested inimmediate andsimultaneous changes inthesynthesis ofallribosomal proteins, toan extent characteristic ofthenewgrowth condition. Nonethe- less, itisnotknownwhether growthcontrol ofyeast ribosomal protein synthesis isregulated transcriptionally or posttranscriptionally. Inaddition, thishighly coordinated andefficient production ofribosomal proteins occurs ina cell inwhichribosomal protein genes arenotclosely linked butaredistributed throughout thegenomeandinwhich someribosomal protein genesoccurinmorethanasingle functional copy(see references 3and27forreviews). Toinvestigate thecontrols whichbring about thestoichi- ometric synthesis ofyeast ribosomal proteins, wefoundit