One μg of purified wild-type RNase R, RNase R R572K, RNase R H456N H565T R572K and RNase II were resolved by 10% SDS-PAGE and visualized by Coomassie blue staining. The molecular masses, in kDa, of protein standards (M) are indicated on the left. Pure RNase II was obtained from Dr. A. Malhotra (University of Miami, Miami, Florida).