Translation during growth and starvation in Saccharomyces cerevisiae
The translation of a series of cat mRNAs containing either the HSP26 5'- leader or various artificial 5'-leaders (Vega Laso et al., 1993) were analysed during growth. From this study, the relative translational efficiencies of these mRNAs were shown to vary from 2% to 100% during mid-exponential phase as observed previously (Vega Laso et al.,1993). However, upon analysing the translation of the various cat constructs during growth, their relative translational efficiencies did not change significantly as yeast cells approached stationary phase. A new set of lacZ mRNAs carrying different natural 5'-leaders (PGK1, PYK1, RpL3, Rp29, GDH1, HSP26, HSP12 and TH14) were constructed. These lacZ mRNAs were placed under the control of the promoters taken from genes expressed during different phases of growth (PGK1 and HSP26). Even though the various PGK1-lacZ and HSP26-lacZ mRNAs were translated differentially, the ability of these mRNAs to compete for the translational apparatus did not appear to change as cells entered stationary phase. The translation of a variety of natural mRNAs encoding a wide range of functions was then analysed by determining their polysomal distribution at various points during growth. Irrespective of the growth phase, a large proportion of each mRNA was detected in the polysomal fractions, suggesting that they continued to be translated in stationary phase. Overall, the data strongly suggest that, under the conditions tested, an excess translational capacity exists in stationary phase yeast cells. Hence gene expression may be largely regulated by transcription upon entry to stationary phase.