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Cellular responses to environmental changes occur on different levels. We investigated the translational response of yeast cells after mild hyperosmotic shock by isolating mRNA associated with multiple ribosomes (polysomes) followed by array analysis. Globally, recruitment of preexisting mRNAs to ribosomes (translational response) is faster than the transcriptional response. Specific functional groups of mRNAs are recruited to ribosomes without any corresponding increase in total mRNA. Among mRNAs under strong translational up-regulation upon shock, transcripts encoding membrane-bound proteins including hexose transporters were enriched. Similarly, numerous mRNAs encoding cytoplasmic ribosomal proteins run counter to the overall trend of down-regulation and are instead translationally mobilized late in the response. Surprisingly, certain transcriptionally induced mRNAs were excluded from ribosomal association after shock. Importantly, we verify, using constructs with intact 5' and 3' untranslated regions, that the observed changes in polysomal mRNA are reflected in protein levels, including cases with only translational up-regulation. Interestingly, the translational regulation of the most highly osmostress-regulated mRNAs was more strongly dependent on the stress-activated protein kinases Hog1 and Rck2 than the transcriptional regulation. Our results show the importance of translational control for fine tuning of the adaptive responses. ; This work was financially supported by the European Commission (QUASI; LSHG-CT2003-530203 and UNICELLSYS; LSHG-CT2007-201142) to P.S and F.P, grants from MICINN and Consolider from the Spanish government, EURYI scheme award (www.esf.org/euryi), ICREA Acadèmia (Generalitat de Catalunya) and Fundación Marcelino Botín (FMB) to F.P, and by grants from the Swedish Research Council (2007-5460) and the Swedish Cancer Fund (09-0772) to P. S.

We analyzed 80 different genomic experiments, and found a positive correlation between both RNA polymerase II transcription and mRNA degradation with growth rates in yeast. Thus, in spite of the marked variation in mRNA turnover, the total mRNA concentration remained approximately constant. Some genes, however, regulated their mRNA concentration by uncoupling mRNA stability from the transcription rate. Ribosome-related genes modulated their transcription rates to increase mRNA levels under fast growth. In contrast, mitochondria-related and stress-induced genes lowered mRNA levels by reducing mRNA stability or the transcription rate, respectively. We also detected these regulations within the heterogeneity of a wild-type cell population growing in optimal conditions. The transcriptomic analysis of sorted microcolonies confirmed that the growth rate dictates alternative expression programs by modulating transcription and mRNA decay. The regulation of overall mRNA turnover keeps a constant ratio between mRNA decay and the dilution of [mRNA] caused by cellular growth. This regulation minimizes the indiscriminate transmission of mRNAs from mother to daughter cells, and favors the response capacity of the latter to physiological signals and environmental changes. We also conclude that, by uncoupling mRNA synthesis from decay, cells control the mRNA abundance of those gene regulons that characterize fast and slow growth. ; Spanish MINECO and European Union funds (FEDER) [BFU2010-21975-C03-01, BFU2013-48643-C3-3-P to J.E.P-O., BFU2010-21975-C03-02, BFU2013-48643-C3-1-P to S.C.]; Regional Valencian Government [GVPROMETEO II 2015/006, ACOMP/2014/253 to J.E.P-O]; Regional Andalusian Government [P12-BIO1938MO to S.C.]; Deutsche Forschungsgemeinschaft [1422/3-1 to Research in the L.M.S lab]; European Research Council Advanced Investigator Grant [AdG-294542]. L.D-R. is a recipient of an FPI fellowship from MINECO. ; Peer Reviewed