Structure of a ribonuclease B+d(pA)4 complex.

The structure of a tetragonal crystal of bovine pancreatic RNase B complexed with d(pA)(4) was determined by molecular replacement and difference Fourier methods. This crystal belongs to space group P4(1)2(1)2 and has unit-cell dimensions a = b = 44.5, c = 156.5 A. The model consists of the enzyme and a tetranucleotide with fractional occupancies, suggesting multiple modes of oligonucleotide binding. It does not include any polysaccharide residues or solvent molecules. After refinement at 2.7 A, the R value was 0.163 with acceptable stereochemistry. The model illustrates a set of well defined interactions for substrate binding, particularly between the central dinucleotide and the enzyme.