The Sso7d DNA-binding protein from Sulfolobus solfataricus has ribonuclease activity

Sso7d is a small, basic, abundant protein from the thermoacidophilic archae on Sulfolobus solfataricus. Previous research has shown that Sso7d can bind double-stranded DNA without sequence specificity by placing its triple-str anded P-sheet across the minor groove. We previously found RNase activity b oth in preparations of Sso7d purified from its natural source and in recomb inant, purified protein expressed in Escherichia coli, This paper provides conclusive evidence that supports the assignment of RNase activity to Sso7d , shown by the total absence of activity in the single-point mutants E35L a nd K12L, despite the preservation of their overall structure under the assa y conditions. In keeping with our observation that the residues putatively involved in RNase activity and those playing a role in DNA binding are loca ted on different surfaces of the molecule, the activity was not impaired in the presence of DNA, If a small synthetic RNA was used as a substrate, Sso 7d attacked both predicted double- and single-stranded RNA stretches, with no evident preference for specific sequences or individual bases. Apparentl y, tile more readily attacked bonds were those intrinsically more unstable. (C) 2001 Federation of European Biochemical Societies. Published by Elsevi er Science B.V. All rights reserved.