Protein Synthesis inLong-Term Stationary-Phase Cultures ofSaccharomyces cerevisiae

We areinterested incharacterizing theprocessofentryintoandthemaintenance ofthestationary phase. Toidentify proteins that areinduced during growth tostationary phase, we examined protein synthesis in long-term stationary-phase cultures usingtwo-dimensional polyacrylamide gelelectrophoresis (2D-PAGE). Although thetotal rateofprotein synthesis declined whengrowthceased after thepostdiauxic phase,the pattern ofproteins synthesized remained similar throughout theexperimental period (28days), exceptatthe diauxic shift. Atthediauxic shift mostproteins detectable by2D-PAGEundergo a transient reduction intheir relative rateofsynthesis thatendswhencells resume growth during thepostdiauxic phase. We conclude from thisthatthetransient repression ofprotein synthesis atthediauxic shift isnotdirectly associated with stationary-phase arrest. Anumberofproteins that aresynthesized after exponential phasehavebeenidentified by2D-PAGE. Theseproteins could bedivided into three temporal classes depending upon whentheir synthesis becamedetectable. Onepostexponential protein, designated p35, was induced later thanallother proteins, and itsrelative rateofsynthesis increased throughout stationary phase. Unlike mostpostexponential proteins, p35 was notregulated byheatshock or glucose repression. We alsoobserved that a direct correlation between steady-state mRNA accumulation andprotein synthesis foranother postexponential protein (Ssa3p) or two closely related constitutive proteins (Ssalp andSsa2p) didnotexist. We conclude fromthisresult that synthesis ofproteins instationary phaseisregulated bymechanisms other thanthecontrol ofsteady-state mRNA accumulation. Cellular growth andproliferation arehighly regulated processes, notonlyinmulticellular organisms, inwhichunregulated cellular proliferation leads toneoplasia, butalsoin microorganisms, inwhichunregulated cellular proliferation mayresult incell death(21). Inmulticellular organisms, growth factors andhormones aretheprimary regulators of cellular proliferation. Inmicroorganisms, suchasthebudding yeast Saccharomyces cerevisiae, theavailability ofnutrients isa majorfactor regulating proliferation. Whenvertebrate cells aredeprived ofcertain growth factors, they ceasegrowth and enteranonproliferating state called Go(2,39). Yeastcells respond tostarvation inasimilar manner, byceasing growth andentering anonproliferating state called stationary phase, which isthought tobesimilar toGo(51). Innature, mostmicroorganisms exist inastationary-phase state induced bynutrient-poor environments (32). Despite the importance ofstationary phase forsurvival innatural environments, itremains apoorly understood period oftheyeast life cycle (51). Themetabolic processes that occurduring starvation-induced stationary phase allow microorganisms tosurvive longperiods without nutrients andmaybeimportant also for resumption ofgrowth should conditions improve. Someofthe physiological changes that occurwhenyeast cells enterstationaryphase areknown(33, 51), butthemolecular mechanisms responsible forentryinto, maintenance of,andexitfrom stationary phase arelargely unknown. Although starvation for different nutrients elicits similar responses (19, 33), itisnot knownwhether all stationary-phase states areidentical. We are interested infurther defining theprocess ofentry intoand maintenance ofstationary phase. Theprocess ofentry into stationary phase isacontinuum of events inresponse toanutrient-depleted environment. The