Evolving precision: rRNA expansion segment 7S modulates translation velocity and accuracy in eukaryal ribosomes.

Ribosome-enhanced translational miscoding of the genetic code causes protein dysfunction and loss of cellular fitness. During evolution, open reading frame length increased, necessitating mechanisms for enhanced translation fidelity. Indeed, eukaryal ribosomes are more accurate than bacterial counterparts, despite their virtually identical, conserved active centers. During the evolution of eukaryotic organisms ribosome expansions at the rRNA and protein level occurred, which potentially increases the options for translation regulation and cotranslational events. Here we tested the hypothesis that ribosomal RNA expansions can modulate the core function of the ribosome, faithful protein synthesis. We demonstrate that a short expansion segment present in all eukaryotes' small subunit, ES7S, is crucial for accurate protein synthesis as its presence adjusts codon-specific velocities and guarantees high levels of cognate tRNA selection. Deletion of ES7S in yeast enhances mistranslation and causes protein destabilization and aggregation, dramatically reducing cellular fitness. Removal of ES7S did not alter ribosome architecture but altered the structural dynamics of inter-subunit bridges thus affecting A-tRNA selection. Exchanging the yeast ES7S sequence with the human ES7S increases accuracy whereas shortening causes the opposite effect. Our study demonstrates that ES7S provided eukaryal ribosomes with higher accuracy without perturbing the structurally conserved decoding center.

Ribosomal RNA expansion segments and their role in ribosome biology.

Ribosomes are universally conserved cellular machines that catalyze protein biosynthesis. The active sites underly immense evolutionary conservation resulting in virtually identical core structures of ribosomes in all domains of life including organellar ribosomes. However, more peripheral structures of cytosolic ribosomes changed during evolution accommodating new functions and regulatory options. The expansion occurred at the riboprotein level, including more and larger ribosomal proteins and at the RNA level increasing the length of ribosomal RNA. Expansions within the ribosomal RNA occur as clusters at conserved sites that face toward the periphery of the cytosolic ribosome. Recent biochemical and structural work has shed light on how rRNA-specific expansion segments (ESs) recruit factors during translation and how they modulate translation dynamics in the cytosol. Here we focus on recent work on yeast, human and trypanosomal cytosolic ribosomes that explores the role of two specific rRNA ESs within the small and large subunit respectively. While no single regulatory strategy exists, the absence of ESs has consequences for proteomic stability and cellular fitness, rendering them fascinating evolutionary tools for tailored protein biosynthesis.