Experimental evolution reveals a general role for the methyltransferase Hmt1 in noise buffering

ABSTRACT Cell-to-cell heterogeneity within an isogenic population has been observed in prokaryotic and eukaryotic cells. Such heterogeneity often manifests at the level of individual protein abundance and may have evolutionary benefits, especially for organisms in fluctuating environments. Although general features and the origins of cellular noise have been revealed, details of the molecular pathways underlying noise regulation remain elusive. Here, we used experimental evolution of Saccharomyces cerevisiae to select for mutations that increase reporter protein noise. By combining bulk segregant analysis and CRISPR/Cas9-based reconstitution, we identified the methyltransferase Hmt1 as a general regulator of noise buffering. Hmt1 methylation activity is critical for the evolved phenotype, and noise buffering is primarily achieved via two Hmt1 methylation targets. Hmt1 functions as an environmental sensor to adjust noise levels in response to environmental cues. Moreover, Hmt1-mediated noise buffering is conserved in an evolutionarily distant yeast species, suggesting broad significance of noise regulation. Author Summary Cell-to-cell heterogeneity within an isogenic population has been observed in prokaryotic and eukaryotic cells. Such heterogeneity often manifests at the level of individual protein abundance and may have evolutionary benefits, especially for organisms in fluctuating environments. Here, we used experimental evolution of Saccharomyces cerevisiae to select for mutations that increase reporter protein noise and identified the methyltransferase Hmt1 as a general regulator of noise buffering. Hmt1 is a central hub protein that is involved in multiple basic cellular pathways, including chromatin remodeling/transcription, translation, ribosome biogenesis, and post-transcriptional regulation. Our results show that Hmt1 constrains the noise level of multiple cellular pathways under normal conditions, so the physiology of individual cells in a population will not deviate too much from optimal peak fitness. However, when cells encounter environmental stresses, HMT1 is quickly down-regulated and expression noise is enhanced to increase the likelihood of population survival. Moreover, the noise buffering function of Hmt1 is conserved in Schizosaccharomyces pombe that diverged from the common ancestor of Saccharomyces cerevisiae more than 400 million years ago. Since the Hmt1 network is conserved from yeast cells to human, it is quite possible that Hmt1-mediated noise buffering also operates in multicellular organisms.