Inhibiting ribosome assembly and ribosome translation have distinctly different effects on the abundance and paralogue composition of ribosomal protein mRNAs in Saccharomyces cerevisiae

Abstract

Many mutations in genes for ribosomal proteins and assembly factors cause cell stress and altered cell fate resulting in congenital diseases, collectively called ribosomopathies. Even though all such mutations depress the cells protein synthesis capacity, they generate many different phenotypes, suggesting that the diseases are not due simply to insufficient protein synthesis capacity. To learn more, we have investigated how the global transcriptome in Saccharomyces cerevisiae responds to reduced protein synthesis generated in two different ways: abolishing the assembly of new ribosomes or inhibiting ribosomal function. Our results show that the mechanism by which protein synthesis is obstructed affects the ribosomal protein transcriptome differentially: ribosomal protein mRNA abundance increases during the abolition of ribosome formation but decreases during the inhibition of ribosome function. Interestingly, the ratio between mRNAs from some, but not all, paralogous genes encoding slightly different versions of a given r-protein change differently during the two types of stress, suggesting that specific ribosomal protein paralogues may contribute to the stress response. Unexpectedly, the abundance of transcripts for ribosome assembly factors and translation factors remains relatively unaffected by the stresses. On the other hand, the state of the translation apparatus does affect cell physiology: mRNA levels for some other proteins not directly related to the translation apparatus also change differentially, though not coordinately with the r-protein genes, in response to the stresses. ImportanceMutations in genes for ribosomal proteins or assembly factors cause a variety of diseases called ribosomopathies. These diseases are typically ascribed to a reduction in the cells capacity for protein synthesis. Paradoxically, ribosomal mutations result in a wide variety of disease phenotypes, even though they all reduce protein synthesis. Here we show that the transcriptome changes differently depending on how the protein synthesis capacity is reduced. Most strikingly, inhibiting ribosome formation and ribosome function have opposite effects on the abundance of mRNA for ribosomal proteins, while genes for ribosome translation and assembly factors show no systematic responses. Thus, the process by which the protein synthesis capacity is reduced contributes decisively to global mRNA composition. This emphasis on process is a new concept in understanding ribosomopathies and other stress responses.