Running rings around RNA: a superfamily of phosphate-dependent RNases.

The exosome of Saccharomyces cerevisiae and the degradosome of Escherichia coli are multienzyme complexes involved in the degradation of mRNA. Both contain enzymes that are similar to the phosphate-dependent exoribonuclease RNase PH. These enzymes are phosphorylases that degrade RNA from the 3'-end. A recent X-ray crystallographic study of the polynucleotide phosphorylase (PNPase) from Streptomyces antibioticus reveals, for the first time, the atomic structure of a member of the RNase PH superfamily. Here, information from the structure of PNPase is used to address two related issues. First, the structure supports the idea that PNPase, which is a trimer of multidomain subunits, arose by duplication of a gene encoding an RNase PH-like enzyme. Second, the structure might explain how RNase PH-like enzymes associate into oligomeric rings that degrade RNA in a processive reaction.