Polyribonucleotide Phosphorylase: An Ancient Enzyme Conserved in Bacteria and Eukarya

Polyribonucleotide phosphorylase (PNPase) is a conserved phosphorolytic exonuclease involved in RNA metabolism in bacteria and eukaryotic organelles. Discovered in the mid-twentieth century, PNPase has since been extensively studied for its role in RNA stability control. PNPase belongs to the PDX protein family, sharing structural similarities with RNase PH and the archaeal and eukaryotic exosomes, and functions as a homotrimer with the three subunits forming a central channel where phosphorolysis occurs. In bacteria, PNPase is a key component of a multiprotein complex—the RNA degradosome—facilitating RNA turnover and quality control. In human mitochondria, it forms the mitochondrial degradosome and contributes to RNA trafficking and decay. Structural analyses reveal a dynamic interplay between its catalytic and RNA-binding domains, allowing PNPase to switch between an open and a closed conformation. Additionally, its activity is modulated, at least in vitro, by small molecules. Despite decades of research, questions remain regarding its precise mechanistic regulation, interactions, and functional diversification.