RNase A (EC 3.1.27.5)

The term ribonuclease (RNase) is an imprecise term and is used to cover both enzymes that cause exonucleolytic cleavage and endonucleolytic cleavage of RNA. Exonucleases may cleave the RNA in 3′-5′ direction or vice versa, whereas some endoribonucleases have a specific requirement for certain bases. For example the RNase from Bacillus cereus cleaves at pyrimidine residues (1 ). Some enzymes produce 5′ phosphomononucleotides, whereas others give rise to 3′ phosphomononucleotides. This chapter focuses on the endoribonuclease RNase A (otherwise described as RNase, RNase I, or pancreatic ribonuclease), which shows some base specificity in where it cleaves RNA. The enzyme has been particularly well characterized at the molecular level.