Ribonucleases H

Abstract

A ribonuclease specific for the RNA strand of a DNA-RNA hybrid was first recognized by Stein and Hausen (1969) and named RNaseH (Hausen and Stein 1970). Other authors have used the term hybridase. Unfortunately, enzymes that degrade the RNA or DNA of hybrids could also be considered hybridases. In this review, discussion is limited to RNaseH as defined by Hausen and Stein.

Many cellular RNasesH have been described but the majority of papers is devoted to the RNaseH activities associated with the viral reverse transcriptases. Despite much study, the metabolic role of these enzymes has remained elusive. A continuing problem is the lack of a uniform nomenclature and a systematic comparison of enzymes characterized in different laboratories.

STRUCTURAL ASPECTS OF DNA-RNA HYBRIDS RELEVANT TO RNaseH ACTIVITY
The characteristic conformation of DNA-RNA hybrids, as exemplified for poly(rA) · poly(dT), is the A form (Fig. 1A). In the A form, the major and minor grooves are similar in size and the bases are relatively inaccessible. The sugar-phosphate backbone, of particular importance for RNaseH substrate specificity, is the most prominent part of the molecule.

DNA-RNA hybrids, unlike DNA-DNA and RNA-RNA duplexes, are polar, with one strand having ribose and the other deoxyribose. This polarity in substrate provides a possible means for the enzyme to move along the substrate in an oriented manner.

The minimum hybrid region necessary to determine susceptibility to an RNaseH is not known precisely. Colicin E1 (Co1E1) plasmid DNA, which contains a small region with ribonucleotide substitutions (Blair et al....