Three ribonucleases H and a reverse transcriptase from the yeast, Saccharomyces cerevisiae

From the yeast, Saccharomyces cerevisiae, three proteins exhibiting ribonuclease H activity were isolated. These proteins differ in molecular weights and enzymatic properties. The two smaller ones, RNAase H(55) and RNAase H(42) are immunologically and structurally related to each other. Neither reacts with antibodies against the largest one, RNAase H(70). Highly purified preparations of RNAase H(70) contain two polypeptides (Mr 70,000 and 160,000) and display reverse transcriptase activity. Deletion of part of the gene for the 160 kDa polypeptide results in mutants possessing about twice the amount of DNA as do wild-type cells. DNA polymerase stimulating activity resides in the 70,000 polypeptide. The processivity of yeast DNA polymerase A(I) does not change in presence of that protein. Possible functions of RNAases H are discussed.