Serological analysis and characterization of calf thymus ribonuclease H IIb

Abstract

Ribonuclease H IIb, which seems to play a physiological role during transcription, was purified from calf thymus tissue. A polyclonal antibody, raised against the most purified ribonuclease H IIb fraction, recognizes in crude extracts almost exclusively a 52‐kDa protein band. By immunoaffinity chromatography and immunoprecipitation experiments, we are able to deplete enzyme extracts from the crossreacting 52‐kDa protein band and from ribonuclease H IIb activity. Enzyme activity is eluted from the immunoaffinity matrix in association with a 52‐kDa protein under denaturing conditions. Immunoaffinity chromatography enables us also to calculate a purification factor of around 20000 from the crude extract. The native molecular mass for the enzyme of around 45 kDa, as determined by gel filtration, suggests that calf thymus ribonuclease H IIb is most probably monomeric. The enzyme possesses an isoelectric point of 7.0. It requires Mg²⁺ ions for activity, is inhibited by N‐ethylmaleimide, and exhibits a pH optimum of 9.0–9.5. The enzyme releases oligoribonucleotides with 3′‐OH and 5′‐phosphate ends, probably in an exonucleolytical manner. The third largest subunit of yeast RNA polymerase A (I) displays ribonuclease H activity [Huet et al. (1976) Nature 261, 431–433]. We discuss our findings in the light of a possible association of ribonuclease H IIb and RNA polymerase A (I) in higher eukaryotes.