A COMMON 40 AMINO-ACID MOTIF IN EUKARYOTIC RNASES H1 AND CAULIMOVIRUSORF-VI PROTEINS BINDS TO DUPLEX RNAS

Eukaryotic RNases H from Saccharomyces cerevisiae, Schizosaccharomyces pombe and Crithidia fasciculata, unlike the related Escherichia coli RNase HI, contain a non-RNase H domain with a common motif, Previously we showed that S.cerevisiae RNase H1 binds to duplex RNAs (either RNA -DNA hybrids or double-stranded RNA) through a region related to the d ouble-stranded RNA binding motif, A very similar amino acid sequence i s present in caulimovirus ORF VI proteins. The hallmark of the RNase H /caulimovirus nucleic acid binding motif is a stretch of 40 amino acid s with 11 highly conserved residues, seven of which are aromatic. Poin t mutations, insertions and deletions indicated that integrity of the motif is important for binding. However, additional amino acids are re quired because a minimal peptide containing the motif was disordered i n solution and failed to bind to duplex RNAs, whereas a longer protein bound well. Schizosaccharomyces pombe RNase H1 also bound to duplex R NAs, as did proteins in which the S. cerevisiae RNase H1 binding motif was replaced by either the C. fasciculata or by the cauliflower mosai c virus ORF VI sequence. The similarity between the RNase H and the ca ulimovirus domain suggest a common interaction with duplex RNAs of the se two different groups of proteins.