Ribonuclease E, an enzyme that processes pre-5S rRNA from its precursor, is now believed to be the major endoribonuclease participating in mRNA turnover in Escherichia coli. The product of the ams/rne/hmp1 gene, which is required for RNase E activity, was overexpressed, purified to near homogeneity by electroelution from an SDS/polyacrylamide gel, and renatured. The purified polypeptide possesses nucleolytic activity in vitro with a specificity identical to that observed for crude RNase E preparations. In addition, both UV crosslinking and RNA-protein blotting unambiguously showed that the Ams/Rne/Hmp1 polypeptide has a high affinity for RNA. Our results demonstrate that RNase E activity is directly attributable to, and is an inherent property of, an RNA-binding protein, the ams/rne/hmp1 gene product.