What is the first action taken by exonucleases in degrading eukaryotic mRNA?

The degradation of eukaryotic mRNA by exonucleases typically starts with the shortening of the poly-A tail. This process is a key step in the mRNA decay pathway, as the poly-A tail plays a crucial role in stabilizing mRNA and facilitating its translation. Once the poly-A tail is gradually shortened, the mRNA becomes more susceptible to degradation by exonucleases that can access the 5' end or the remaining RNA body.

The shortening of the poly-A tail signals that the mRNA is no longer needed, leading to its eventual degradation. This action is crucial in regulating gene expression, as it ensures that mRNA is only present when necessary for protein synthesis. By stabilizing the mRNA initially, the poly-A tail supports translation and prevents unnecessary degradation, but its shortening marks the transition towards mRNA decay.

Understanding this process is key to appreciating how cells regulate the life cycle of their mRNA and ensure timely responses to changing cellular environments or developmental cues.