Exoribonuclease II (RNase II), encoded by thernb gene, is a ubiquitous enzyme that is responsible for 90% of the hydrolytic activity in Escherichia colicrude extracts. The E. colistrain SK4803, carrying the mutant allelernb296, has been widely used in the study of the role of RNase II. We determined the DNA sequence ofrnb296 and cloned this mutant gene in an expression vector. Only a point mutation in the coding sequence of the gene was detected, which results in the single substitution of aspartate 209 for asparagine
