Hydrolysis of the 5′-p-nitrophenyl ester of TMP by oligoribonucleases (ORN) from Escherichia coli, Mycobacterium smegmatis, and human

Escherichia coli oligoribonuclease (EcoORN), encoded by the orn gene, is a 3′-5′ exonuclease that degrades short single-stranded oligoribonucleotides to rNMPs in the final step of RNA degradation. The orn gene is essential in E. coli, but not in higher organisms, and close homologues are present in other genomes from the β and γ subdivisions of the Protobacteriaceae, including many pathogenic species. We report here the expression in E. coli of orn and homologues from Mycobacterium smegmatis and human, and large-scale purification of the three enzymes. All three were found to promote the hydrolysis of the 5′-p-nitrophenyl ester of TMP (pNP-TMP) with similar values of Michaelis-Menten parameters (k_cat = 100-650 min^-1, K_M = 0.4-2.0 mM, at pH 8.00 and 25 °C, with 1 mM Mn²⁺). Hydrolysis by pNP-TMP by all three enzymes depended on a divalent metal ion, with Mn²⁺ being preferred over Mg²⁺ as cofactor, and was inhibited by Ni²⁺. The concentration dependency of Mn²⁺ was examined, giving K_Mn values of 0.2-0.6 mM. The availability of large amounts of the purified enzymes and a simple spectrophotometric assay for ORN activity should facilitate large-scale screening for new inhibitors of bacterial oligoribonucleases.