Translational control through ribosome heterogeneity and functional specialization.

The conceptual origins of ribosome specialization can be traced back to the earliest days of molecular biology. Yet, this field has only recently begun to gather momentum, with numerous studies identifying distinct heterogeneous ribosome populations across multiple species and model systems. It is proposed that some of these compositionally distinct ribosomes may be functionally specialized and able to regulate the translation of specific mRNAs. Identification and functional characterization of specialized ribosomes has the potential to elucidate a novel layer of gene expression control, at the level of translation, where the ribosome itself is a key regulatory player. In this review, we discuss different sources of ribosome heterogeneity, evidence for ribosome specialization, and also the future directions of this exciting field. Over the past decade, it has become increasingly apparent that ribosomes are significantly more heterogeneous than originally thought, with variation emanating from both the rRNA and protein content of ribosomes. This heterogeneity can potentially confer functional ribosome specialization, contributing to translational control, thereby identifying the ribosome as a key regulatory player in translation. Studies have uncovered interindividual and intertissue variant rRNA alleles and there has also been a recent focus on identifying functional roles for the previously enigmatic eukaryotic rRNA expansion segments. Technological advances have permitted accurate, quantitative analysis of rRNA modifications across several organisms, with functionality ascribed to certain rRNA modifications. Other work has centered on identifying specialization arising due to changes in ribosome protein composition. [ABSTRACT FROM AUTHOR]