Redefining the Translational Status of 80S Monosomes.

Fully assembled ribosomes exist in two populations: polysomes and monosomes. While the former has been studied extensively, to what extent translation occurs on monosomes and its importance for overall translational output remain controversial. Here, we used ribosome profiling to examine the translational status of 80S monosomes in Saccharomyces cerevisiae. We found that the vast majority of 80S monosomes are elongating, not initiating. Further, most mRNAs exhibit some degree of monosome occupancy, with monosomes predominating on nonsense-mediated decay (NMD) targets, upstream open reading frames (uORFs), canonical ORFs shorter than ∼ 590 nt, and ORFs for which the total time required to complete elongation is substantially shorter than that required for initiation. Importantly, mRNAs encoding low-abundance regulatory proteins tend to be enriched in the monosome fraction. Our data highlight the importance of monosomes for the translation of highly regulated mRNAs.