Genome-wide polysomal analysis of a yeast strain with mutated ribosomal protein S9

The yeast ribosomal protein S9 (S9) is located at the entrance tunnel of the mRNA into the ribosome. It is known to play a role in accurate decoding and its bacterial homolog (S4) has recently been shown to be involved in opening RNA duplexes. Here we examined the effects of changing the C terminus of S9, which is rich in acidic amino acids and extends out of the ribosome surface.

We performed a genome-wide analysis to reveal effects at the transcription and translation levels of all yeast genes. While negligible relative changes were observed in steady-state mRNA levels, a significant number of mRNAs appeared to have altered ribosomal density. Notably, 40% of the genes having reliable signals changed their ribosomal association by more than one ribosome. Yet, no general correlations with physical or functional features of the mRNA were observed. Ribosome Density Mapping (RDM) along four of the mRNAs with increased association revealed an increase in ribosomal density towards the end of the coding region for at least two of them. Read-through analysis did not reveal any increase in read-through of a premature stop codon by the mutant strain.

The ribosomal protein rpS9 appears to be involved in the translation of many mRNAs, since altering its C terminus led to a significant change in ribosomal association of many mRNAs. We did not find strong correlations between these changes and several physical features of the mRNA, yet future studies with advanced tools may allow such correlations to be determined. Importantly, our results indicate an accumulation of ribosomes towards the end of the coding regions of some mRNAs. This suggests an involvement of S9 in ribosomal dissociation during translation termination.