SaRD, a new protein isolated from the extremophile archaeon Sulfolobus acidocaldarius, is a thermostable ribonuclease with DNA-binding properties.

We have isolated the thermostable 9 kDa SaRD-protein from Sulfolobus acidocaldarius which exhibit RNase activity as well as DNA-binding properties (SaRD). The amino acid composition and the sequence of the 16 N-terminal amino acids show similarities to different RNases as well as to DNA-binding proteins from thermophilic archea. The RNase activity was demonstrated by 5S rRNA degradation, thin layer chromatography and a zymogram. The temperature optimum for the RNase activity is 65 degrees C. The pH optimum ranges from 6.5-7.0. DNA-binding properties were shown by gel-shift assays on agarose gels. In a similar way SaRD mediated protection of DNA against DNase I digestion and Sau3A I restriction could be demonstrated. The melting point (Tm) of genomic DNA was raised from 68 degrees C to 90 degrees C by addition of the SaRD-protein. CD spectroscopy indicated that SaRD is very stable near neutral pH and can neither be unfolded by temperatures up to 85% C nor by addition of 8 M urea.