Preferential Ty1 retromobility in mother cells and nonquiescent stationary phase cells is associated with increased concentrations of total Gag or processed Gag and is inhibited by exposure to a high concentration of calcium.

Andrew C Peifer,Patrick H Maxwell

doi:10.18632/aging.101402

Abstract

Retrotransposons are abundant mobile DNA elements in eukaryotic genomes that are more active with age in diverse species. Details of the regulation and consequences of retrotransposon activity during aging remain to be determined. Ty1 retromobility in Saccharomyces cerevisiae is more frequent in mother cells compared to daughter cells, and we found that Ty1 was more mobile in nonquiescent compared to quiescent subpopulations of stationary phase cells. This retromobility asymmetry was absent in mutant strains lacking BRP1 that have reduced expression of the essential Pma1p plasma membrane proton pump, lacking the mRNA decay gene LSM1, and in cells exposed to a high concentration of calcium. Mother cells had higher levels of Ty1 Gag protein than daughters. The proportion of protease-processed Gag decreased as cells transitioned to stationary phase, processed Gag was the dominant form in nonquiescent cells, but was virtually absent from quiescent cells. Treatment with calcium reduced total Gag levels and the proportion of processed Gag, particularly in mother cells. We also found that Ty1 reduced the fitness of proliferating but not stationary phase cells. These findings may be relevant to understanding regulation and consequences of retrotransposons during aging in other organisms, due to conserved impacts and regulation of retrotransposons.

Highlights

Retrotransposons copy their RNA transcripts through reverse transcription to produce cDNA molecules that are inserted into genomes, and they have been associated with aging in several model organisms [1]
Ty1 retromobility in Saccharomyces cerevisiae is more frequent in mother cells compared to daughter cells, and we found that Ty1 was more mobile in nonquiescent compared to quiescent subpopulations of stationary phase cells
Since Ty1 mRNA and Gag-GFP levels are similar in mother and daughter cells but cDNA levels are much higher in mother cells [5], we tested whether candidate genes contributing to asymmetries between mothers and daughters that could influence intermediate steps in the Ty1 retromobility cycle regulate Ty1 retromobility asymmetry

Summary

Introduction

Retrotransposons copy their RNA transcripts through reverse transcription to produce cDNA molecules that are inserted into genomes, and they have been associated with aging in several model organisms [1]. Retrotransposon expression, integration of new cDNA copies (retrotransposition), or both are increased with age in S. cerevisiae, C. elegans, D. melanogaster, mice, and normal diploid human cells [2]. A protease domain in Pol processes the initial p49 form of Gag into a p45 form, cleaves Gag from Gag-Pol, and processes Pol into protease, integrase, and reverse transcriptase/ RNase H domains during VLP maturation [8]. Reverse transcriptase/RNase H synthesizes a cDNA from Ty1 mRNA in VLPs that can be integrated at a new genomic site by the integrase domain [8]. Increased Ty1 retrotransposition frequency in aging mother cells relative to their daughter cells was not found to be due to substantial asymmetry in Ty1 mRNA or Gag accumulation in mother cells versus their daughter cells, but was correlated with a large increase in Ty1 cDNA in mothers compared to daughters [5]

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Conclusion