The 62 residue peptide, SSR(1–62), whose sequence corresponds to that of ribonuclease (RNase) from Sulfolobus solfataricus, and its related peptides, SSR(1–22) and SSR(10–62), were chemically synthesized and their RNase activity and DNA-binding activity were examined. The RNase activity assay using yeast RNA or tRNAfMet as substrate showed that the synthetic peptide SSR(1–62) did not hydrolyze yeast RNA or tRNAfMet. These data were not consistent with previous reports that both the native peptide isolated from S. solfataricus [Fusi et al. (1993) Eur. J. Biochem. 211, 305–311] and the recombinant peptide expressed in Escherichia coli [Fusi et al. (1995) Gene 154, 99–103] were able to hydrolyze tRNAfMet. However, the synthetic SSR(1–62) exhibited DNA-binding activity. In the presence of synthetic SSR(1–62), the cleavage of DNA (plasmid pUCRh2-4) by restriction endonuclease (EcoRI) was not observed, suggesting that synthetic SSR(1–62) bound to DNA protected DNA from its enzymatic digestion. Neither SSR(1–22) nor SSR(10–62) prevented DNA from being cleaved by a restriction enzyme. These findings strongly suggest the importance of not only the N-terminal region of SSR(1–62) but also the C-terminal region for DNA-binding. Circular dichroism spectroscopy of synthetic SSR(1–62) indicated a β-sheet conformation, in contrast with synthetic SSR(1–22), which exhibited an unordered conformation.
