Time-dependent increase in ribosome processivity

We created a novel tripartite reporter RNA to separately and simultaneously examine ribosome translation rates at the 5'- and 3'-ends of a large open reading frame (ORF) in vitro in HeLa cell lysates. The construct contained Renilla luciferase (RLuc), beta-galactosidase and firefly luciferase (FLuc) ORFs linked in frame and separated by a viral peptide sequence that causes cotranslational scission of emerging peptide chains. The length of the ORF contributed to low ribosome processivity, a low number of initiating ribosomes completing translation of the entire ORF. We observed a time-dependent increase in FLuc production rate that was dependent on a poly(A) tail and poly(A)-binding protein, but was independent of eIF4F function. Stimulation of FLuc production occurred earlier on shorter RNA templates. Cleavage of eIF4G at times after ribosome loading on templates occurred did not cause immediate cessation of 5'-RLuc translation; rather, a delay was observed that shortened when shorter templates were translated. Electron microscopic analysis of polysome structures in translation lysates revealed a time-dependent increase in ribosome packing and contact that correlated with increased processivity on the FLuc ORF. The results suggest that ORF transit combined with PABP function contribute to interactions between ribosomes that increase or sustain processivity on long ORFs.