Identification of a yeast ribonuclease H as an Sm antigen

We have isolated a 55-kDa enzyme from Saccharomyces cerevisiae on the basis of its ability to hydrolyze specifically the RNA moiety of RNA/DNA hybrids [RNase H(55)]. Remarkably, monospecific anti-[RNase H(55)] antibodies revealed that the protein associates with several small RNAs, including some of the essential yeast spliceosomal snRNAs. Moreover, immunoprecipitation as well as immunoblotting experiments demonstrated that the yeast enzyme reacts (a) with human anti-Sm autoantisera, (b) with a monoclonal antibody specific for the human snRNP proteins B/B', but (c) not with U1-ribonucleoprotein-specific autoantibodies. These results disclosed a hitherto unexpected degree of evolutionary conservation in snRNP protein structure between yeast and man. Additionally, our findings suggested a re-evaluation of the enzymatic mechanism of RNases H which recognize both RNA and RNA/DNA hybrids.