INTRODUCTION. Ribonucleases (RNases) play a central role in all cellular RNA processes. These processes include mRNA degradation, and maturation and turnover of stable RNAs, which are vital for the proper functioning of all cells. E. coli has served as a model system for understanding the role of ribonucleases in RNA metabolism, and eight distinct exoribonucleases have been identified in this bacterium. Of these, three (RNase T, RNase D, and oligoribonuclease) are members of a larger exonuclease superfamily (named the DEDD exonuclease family, after the four invariant acidic residues in these proteins) that includes the proof-reading domains of DNA polymerases[1].