Abstract
Fine-tuned regulation of protein biosynthesis is crucial for cellular fitness and became even more vital when cellular and organismal complexity increased during the course of evolution. In order to cope with this augmented demand for translation control, eukaryal ribosomes have gained extensions both at the ribosomal protein and rRNA levels. Here we analyze the functional role of ES27L, an rRNA expansion segment in the large ribosomal subunit of Saccharomyces cerevisiae. Deletion of the b-arm of this expansion segment, called ES27Lb, did not hamper growth during optimal conditions, thus demonstrating that this 25S rRNA segment is not inherently crucial for ribosome functioning. However, reductive stress results in retarded growth and rendered unique protein sets prone to aggregation. Lack of ES27Lb negatively affects ribosome-association of known co-translational N-terminal processing enzymes which in turn contributes to the observed protein aggregation. Likely as a compensatory response to these challenges, the truncated ribosomes showed re-adjusted translation of specific sets of mRNAs and thus fine-tune the translatome in order to re-establish proteostasis. Our study gives comprehensive insight into how a highly conserved eukaryal rRNA expansion segment defines ribosomal integrity, co-translational protein maturation events and consequently cellular fitness.
Highlights
The ribosome is a large, complex ribonucleoprotein (RNP) assembly that is responsible for protein biosynthesis in all domains of life
To verify that the entire population of ribosomes in the ES27Lb strain originate from the pTET plasmid after plasmid exchange and as a consequence truly lacks the expansion segment ES27Lb, a modified primer extension reaction (Morgan analysis) was performed on total RNA isolated from the cells grown in YPD + Hygromycin B (HygB)
Spliced HAC1 mRNA is actively translated to the transcription factor that activates further genes downstream of the unfolded protein response (UPR) pathway [31]
Summary
The ribosome is a large, complex ribonucleoprotein (RNP) assembly that is responsible for protein biosynthesis in all domains of life It is highly conserved at its central functional cores such as the decoding center and the peptidyl transferase center (PTC). One of the striking differences between prokaryal and eukaryal ribosomes is the size – over the course of evolution the ribosome has expanded in response to the increasing functional and regulatory demands of the cell, whilst conserving the central core. These expansions have occurred in both subunits of the ribosome, in both the rRNA and associated ribosomal proteins (RP).
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