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

Stephen J. Kim,Randy Strich

doi:10.1186/s13008-016-0024-3

Abstract

BackgroundThe 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.ResultsThis 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.ConclusionsThese 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.

Highlights

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
Similar to a previous report [18], Rpl22A and/or Rpl22B function is not required for mitotic cell division in rich medium the doubling times for the rpl22A∆ and rpl22∆ double mutant strains were slightly reduced compared to wild type or rpl22B∆ cultures
We found that rpl22A∆ or rpl22∆ strains were unable to form colonies when incubated at 4 °C for 24 h returned to growth at 25° (Fig. 1a)

Summary

Introduction

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 Ume. In response to poor nitrogen sources, haploid and diploid yeast will undergo a dimorphic switch leading to invasive or pseudohyphal growth, respectively. The switch from mitotic to meiotic cell divisions requires expression of IME1, which induces the meiotic transcription program by binding and triggering the destruction of the Ume repressor [6]. IME1 transcription is controlled by a complex and extensive set of cis-acting promoter elements that respond to cell type, carbon and nitrogen signals [8, 9]

Methods

Results

Conclusion