Head and Embryo 80S monosomes are much more translationally active than 80S in other tissues

Translational regulation at the stage of initiation impacts the number of ribosomes translating each mRNA molecule. For example, multiple ribosomes can engage on a single mRNA forming a polysome, resulting in highly efficient protein synthesis. However, the translational activity of single 80S ribosomes on mRNA (monosomes) is less well understood, even though these 80S monosomes represent the dominant ribosomal complexes in many tissues. 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 uORFs. Overall, these findings highlight that regulation of translation initiation, and therefore the number of ribosomes bound per mRNA, varies substantially across tissues. Overall design: In order to investigate differential transcript enrichment in different ribosome fractions (80S vs Polysomes), we performed Frac-seq analysis on purified 80S and polysome fractions from the flies' heads in biological triplicates for each sample group. Total RNA-seq from the heads was also performed as a control