Abstract
Crystals of a complex formed between ribonuclease A and d(pA)4 were grown and their structure determined by a combination of multiple isomorphous replacement (MIR) and molecular replacement techniques. The known structure of ribonuclease A in the correct orientation in the unit cell yielded a conventional crystallographic R factor of 0.32 at 2.8 A resolution when refined as a rigid body. Difference Fourier syntheses permitted determination of the disposition of the DNA in the unit cell. Refinement of both protein and DNA by constrained-restrained least squares procedures resulted in an R factor of 0.22 at 2.5 A resolution. The structure of the crystalline complex is comprised of four ordered oligomers of d(pA)4 associated with each molecule of RNAse. If the sites of interaction between protein and d(pA)4 fragments are mapped on the surface of the protein, they describe an essentially continuous path into and through the active site, across the surface of the enzyme and finally into the basic amino acid cluster on the opposite side of the protein.
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