The space between notes: emerging roles for translationally silent ribosomes.

In addition to their central functions in translation, ribosomes can adopt inactive structures that are fully assembled yet devoid of mRNA. We describe how the abundance of idle eukaryotic ribosomes is influenced by a broad range of biological conditions spanning viral infection, nutrient deprivation, and developmental cues. Vacant ribosomes may provide a means to exclude ribosomes from translation while also shielding them from degradation, and the variable identity of factors that occlude ribosomes may impart distinct functionality. We propose that regulated changes in the balance of idle and active ribosomes provides a means to fine-tune translation. We provide an overview of idle ribosomes, describe what is known regarding their function, and highlight questions that may clarify their biological roles.