Translational activity of 80S monosomes varies dramatically across different tissues.

Translational regulation at the stage of initiation can impact the number of ribosomes translating each mRNA molecule. However, the translational activity of single 80S ribosomes (monosomes) on mRNA is less well understood, even though these 80S monosomes represent the dominant ribosomal complexes in vivo. Here, we used cryo-EM to determine the translational activity of 80S monosomes across different tissues in Drosophila melanogaster. We discovered that while head and embryo 80S monosomes are highly translationally active, testis and ovary 80S monosomes are translationally inactive. RNA-Seq analysis of head monosome- and polysome-translated mRNAs, revealed that head 80S monosomes preferentially translate mRNAs with TOP motifs, short 5'-UTRs, short ORFs and are enriched for the presence of uORFs. Overall, these findings highlight that regulation of translation initiation and protein synthesis is mostly performed by monosomes in head and embryo, while polysomes are the main source of protein production in testis and ovary.