Binding of oligonucleotides to the disk of tobacco-mosaic-virus protein.

The trinucleoside diphosphate A-A-G and the hexanucleotide fraction from a ribonuclease I digest of yeast RNA have been soaked into crystals of the disk aggregate of tobacco mosaic virus protein. At high concentrations these cause disruption of the crystal, probably by mimicking the normal nucleation of assembly. At lower nucleotide concentrations the crystals remain intact and the differences caused by nucleotide binding have been studied by X-ray diffraction. The most obvious change is an upwards movement of about 0.3 nm at the low-radius end of the left radial helix in the protein with some stiffening of the helix so that it now extends visibly in from 4 nm to 6 nm radius. Similar shifts also occur in the right radial and left slewed helices. A positive peak, which is tentatively identified with the bound oligonucleotide, is seen around 4 nm radius and below the right radial helix. The amino acid residues in possible contact with this feature are discussed.