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Background  The transition from mitotic cell division to meiotic development in  S. cerevisiae  requires induction of a transient transcription program that is initiated by Ime1-dependent destruction of the repressor Ume6. Although  IME1  mRNA is observed in vegetative cultures, Ime1 protein is not suggesting the presence of a regulatory system restricting translation to meiotic cells.    Results  This study demonstrates that  IME1  mRNA translation requires Rpl22A and Rpl22B, eukaryotic-specific ribosomal protein paralogs of the 60S large subunit. In the absence of Rpl22 function,  IME1  mRNA synthesis is normal in cultures induced to enter meiosis. However, Ime1 protein production is reduced and the Ume6 repressor is not destroyed in  rpl22  mutant cells preventing early meiotic gene induction resulting in a pre-meiosis I arrest. This role for Rpl22 is not a general consequence of mutating non-essential large ribosomal proteins as strains lacking Rpl29 or Rpl39 execute meiosis with nearly wild-type efficiencies. Several results indicate that Rpl22 functions by enhancing  IME1  mRNA translation. First, the Ime1 protein synthesized in  rpl22  mutant cells demonstrates the same turnover rate as in wild-type cultures. In addition,  IME1  transcript is found in polysome fractions isolated from  rpl22  mutant cells indicating that mRNA nuclear export and ribosome association occurs. Finally, deleting the unusually long 5′UTR restores Ime1 levels and early meiotic gene transcription in  rpl22  mutants suggesting that Rpl22 enhances translation through this element. Polysome profiles revealed that under conditions of high translational output, Rpl22 maintains high free 60S subunit levels thus preventing halfmer formation, a translation species indicative of mRNAs bound by an unpaired 40S subunit. In addition to meiosis, Rpl22 is also required for invasive and pseudohyphal growth.    Conclusions  These findings indicate that Rpl22A and Rpl22B are required to selectively translate  IME1  mRNA that is required for meiotic induction and subsequent gametogenesis. In addition, our results imply a more general role for Rpl22 in cell fate switches responding to environmental nitrogen signals.

Rpl22 is required for IME1 mRNA translation and meiotic induction in S. cerevisiae