11 Ribonucleases H

Abstract

I. INTRODUCTION
Ribonuclease H (RNase H) (Ribonucleate [in RNA:DNA hybrids] 5′-oligonucleotidohydrolase, E.C.3.1.26.4) specifically cleaves the RNA strand in an RNA:DNA hybrid duplex. The designation H (for hybrid) was introduced by Hausen and Stein (1970) to describe a new ribonuclease activity discovered in extracts from calf thymus (Stein and Hausen 1969). All known nucleases with RNase H activity require divalent metal cations for hydrolysis of the RNA phosphodiester bond producing 5′-phosphate and 3′-hydroxyl termini. The presence of the free 3′-hydroxyl terminus makes the resulting RNA fragment suitable as a primer for DNA polymerase.

RNases H comprise a family of ubiquitous enzymes, detected in multiple forms in all organisms from bacteria to mammals. However, with the exception of their well-understood function in the life cycle of retroviruses, the biological roles of other cellular RNases H are much less clear. In addition to their presumed involvement in DNA replication (and the practical usefulness of the isolated enzyme as a reagent for in vitro manipulations with nucleic acids), two other factors with possible therapeutic implications have contributed to the recent increased interest in RNases H: (1) the presumed role of endogenous RNase H activity in the mechanism of action of antisense oligonucleotides and (2) the potential importance of the enzyme as an antiviral target, especially in connection with human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS).

Since the first comprehensive overview of RNases H by Crouch and Dirksen (1982), two more recent reviews have appeared (Crouch 1990; Wintersberger 1990) that deal with...