Oligo(uridine) tailing and exonucleolytic decay drive 40S ribosome degradation in response to stress

Inside every living cell, tiny machines called ribosomes work tirelessly to build the proteins that keep life moving. These machines are not simple tools; they are complex structures made of both proteins and a specific type of genetic material known as ribosomal RNA. When a cell faces harsh conditions, such as a lack of food, it must conserve energy by shutting down these factories. To do this efficiently, the cell does not just turn them off; it dismantles them completely, breaking down the large, stable structures into their basic parts so they can be recycled or discarded. Scientists have long known that a specific protein, acting like a molecular tagger, helps mark these ribosomes for destruction, but the exact steps the cell takes to chew up the RNA core of the machine remained a mystery.

A new study reveals the precise mechanism the cell uses to dismantle these ribosomes when starved. Researchers found that a protein called RIOK3 acts as the coordinator for this cleanup crew. Once the cell senses starvation, RIOK3 binds to the ribosomes that have been marked for removal and recruits two other key workers: an enzyme that adds a string of uridine molecules to the end of the RNA, and another enzyme that acts as a shredder. The first worker, known as TUT7, attaches a tail made of multiple uridine units to the very end of the 18S RNA component within the ribosome. This addition of a uridine tail serves as a signal that changes the nature of the RNA, making it recognizable to the second worker, an enzyme called DIS3L2.

Once the tail is in place, DIS3L2 attaches to the modified RNA and begins to eat away at it from the end, breaking the long strand down into smaller pieces in a process known as decay. The researchers observed that this process is not always a single, smooth event. Sometimes, as the shredder works, it leaves behind fragments that are still too large to be discarded. These leftover pieces undergo the same process again, receiving new uridine tails and being attacked by the shredder once more. This cycle of adding tails and cutting them down continues until the RNA is fully broken apart.

The importance of this specific pathway was confirmed when the researchers removed the shredding enzyme from the system. Without DIS3L2, the ribosomes failed to break down properly during starvation. Instead of disappearing, the 18S RNA accumulated in the cell, covered in the uridine tails that had been added but never processed. This finding demonstrates that the addition of these oligo-uridine tails is not just a side effect but the driving force that allows the cell to degrade its ribosomes. By defining this step-by-step process, the study provides a clear picture of how cells manage the complex task of taking apart their own machinery when resources run low.