Ribonuclease H from chick embryos cleaves precisely at the junction between the RNA and DNA portion of the hybrid helix

Evidence from CD spectra that d(purine).r(pyrimidine) and r(purine).d(pyrimidine) hybrids are in different structural classes

CD spectra and difference CD spectra of four d(oligopurine).r(oligopyrimidine) and four r(oligopurine).d(oligopyrimidine) hybrid duplexes containing mixed A.T(U) and G.C base pairs were compared with the spectra of four DNA.DNA and four RNA.RNA oligomer duplexes of similar repeating sequences. The 16 duplexes were formed by mixing oligomers that were 24 nucleotides long. The buffer was 0.05 M Na+ (phosphate), pH 7.0. DNA.DNA and RNA.RNA oligomer duplexes were used as reference B-form and A-form structures. We found that the CD spectra of d(purine).r(pyrimidine) and r(purine).d(pyrimidine) hybrid duplexes were different from the CD spectra of either DNA.DNA or RNA.RNA duplexes. The data suggested that these hybrids have intermediate structures between A-form RNA and B-form DNA structures. The CD spectra of d(purine).r(pyrimidine) and r(purine).d(pyrimidine) hybrid duplexes were different from each other, but the hybrids in each class had consistent CD spectra as indicated by nearest-neighbor comparisons. Thus, it appeared that the two types of hybrids belonged to different structural classes. The negative 210 nm band found in difference CD spectra was correlated with the presence of an r(purine) strand in the hybrid duplexes. The melting temperatures (Tm values) of these hybrids were compared with the Tm values of the DNA.DNA and RNA.RNA duplexes. The order of the thermal stability was: RNA.RNA duplex > r(purine).d(pyrimidine) hybrid > DNA.DNA duplex > d(purine).r(pyrimidine) hybrid, when comparing analogous sequences.

Ribonucleases H of retroviral and cellular origin

Ribonucleases H (RNases H) are enzymes which catalyse the hydrolysis of the RNA-strand of an RNA-DNA hybrid. Retroviral reverse transcriptases possess RNase H activity in addition to their RNA- as well as DNA-dependent DNA-polymerizing activity. These enzymes transcribe the viral single stranded RNA-genome into double stranded DNA, which then can be handled by the host cell like one of its own genes. Various, sometimes highly repeated, sequences related to retroviruses and like these encompassing two separate domains, one of which potentially codes for a DNA polymerizing, the other for an RNase H activity, are found in genomes of uninfected cells. In addition proteins coded for by cellular genes (e.g. from E. coli and from yeast) are known, which exhibit RNase H activity, the biological function of which is not fully understood. In the light of these facts the question of whether retroviral RNases H could be promising targets for antiviral drugs is discussed.

Activity of ribonuclease H in cells of chronic B-lymphocytic leukaemia: correlation with clinical stage

Peripheral blood mononuclear cells (PBMNC) from 23 healthy subjects and 39 patients with B-cell chronic leukaemia (B-CLL) were assayed for ribonuclease H activity using as substrate the filter-immobilized synthetic homopolymer hybrid 3H-poly(rA):poly(dT). In 69% of the leukaemia patients examined enzyme activities were above those estimated in cells from the healthy controls. The mean enzyme levels for the normal and the leukaemic samples group were (per cent substrate hydrolysis): 8.1 +/- 8.9 and 58.7 +/- 40.8, respectively, their difference being statistically highly significant (P less than 0.0001). This result does not represent homogeneity within the cells but is due to a subclass of cells within the leukaemic clone containing the enzyme, thus contributing through pool size fluctuation to the wide variations of enzyme activity observed among the patients. These cells containing high activity could not be identified with either the prolymphocytes or the large lymphocytes. The activity of ribonuclease H in the examined CLL patients correlated with disease stage (Binet) (P = 0.011) and appeared to serve as a sensitive indicator of disease progression when compared with a number of other known prognostic parameters.

Yeast ribonuclease H(70) cleaves RNA-DNA junctions

A specific substrate, M13 DNA:RNA-[32P]DNA, was synthesized to investigate the mode of cleavage of enzymes with RNase H activity. RNase H(70) from Saccharomyces cerevisiae hydrolyzes the phosphodiester bond at the RNA-DNA junction of this substrate, thereby producing a 5'-monophosphate-terminated polydeoxyribonucleotide and 3'-hydroxyl-terminated oligoribonucleotides.