Sulfur-dependent translational regulation via methylation multiplicity of 18S rRNA

N6-methyladenosine (m6A) is a conserved nucleoside modification that regulates many facets of RNA metabolism and cellular physiology. Using quantitative mass spectrometry, we find that the universally conserved tandem adenosines at the 3' end of 18S rRNA (A1781 and A1782 in Saccharomyces cerevisiae), which were thought to be constitutively di-methylated (i.e. N6, N6-dimethyladenosine or m62A), are also mono-methylated (i.e. m6A). Although present at substoichiometric amounts, m6A at these positions increases significantly and specifically in response to sulfur starvation in both yeast and mammalian cells. Combining yeast genetics and ribosome-profiling, we further demonstrate that ribosomes bearing different numbers of methyl groups (zero, one, and two) at these tandem adenosines exhibit distinct translation profiles in a sulfur-dependent fashion. Our work thus reveals methylation multiplicity as a mechanism to regulate translation and also uncovers the functional importance of the ubiquitous m62A modification in 18S rRNA. Overall design: We performed ribosome profiling and RNA-seq on three Saccharomyces cerevisiae strains growing with and without methionine