%0 Journal Article %A Nikolai Slavov %A Stefan Semrau %A Edoardo Airoldi %A Bogdan Budnik %A Alexander van Oudenaarden %T Differential stoichiometry among core ribosomal proteins %D 2015 %R 10.1101/005553 %J bioRxiv %P 005553 %X Understanding the regulation and structure of ribosomes is essential to understanding protein synthesis and its dysregulation in disease. While ribosomes are believed to have a fixed stoichiometry among their core ribosomal proteins (RPs), some experiments suggest a more variable composition. Testing such variability requires direct and precise quantification of RPs. We used mass-spectrometry to directly quantify RPs across monosomes and polysomes of mouse embryonic stem cells (ESC) and budding yeast. Our data show that the stoichiometry among core RPs in wild-type yeast cells and ESC depends both on the growth conditions and on the number of ribosomes bound per mRNA. Furthermore, we find that the fitness of cells with a deleted RP-gene is inversely proportional to the enrichment of the corresponding RP in polysomes. Together, our findings support the existence of ribosomes with distinct protein composition and physiological function. %U https://www.biorxiv.org/content/biorxiv/early/2015/09/19/005553.full.pdf