center The basic diffuse scattering features from yeast initiator tRNA have been described previously [Glover et al., 1991][Kolatkar et al., 1990]. Preliminary analysis of this diffuse intensity has been carried out to obtain specific details about the disorder [Kolatkar et al., 1992]. The data used in the current analysis have been collected on imaging plates using synchrotron radiation at the Photon Factory (Japan) [Basavappa \& Sigler, 1991]. The diffraction from these crystals reveals various forms of diffuse scattering. The most striking of these features are the streaks seen in the diffraction images in Figures 2a and 3a.
The orientation of the streaks of diffuse scattering can be correlated with the packing of molecules in the crystal. Figure 2a shows an image of tRNA X-ray scattering which corresponds to the unit cell orientation shown in Figure 2c. In the diffraction image, the c axis runs from the lower left corner to the upper right corner. In the unit cell drawing, the c axis runs in the same direction. Similarly, Figure 3a shows the diffraction data corresponding to the unit cell orientation in Figure 3c. In both views, the diffuse streaks run perpendicular to the c axis and also to the pseudo-helix lying along the c axis. If some part of the molecule were moving parallel to the c axis, diffuse scattering situated along the c axis would be expected. It is possible to estimate the correlation distances for this component of the diffuse scattering by judging the dimensions of the streaks in reciprocal Angstroms and inverting. To refine these correlation distances, the diffuse scattering from such a model is calculated and compared to the actual diffraction data. The correctness of the model is determined by the degree of similarity between the calculated and observed diffuse scattering.
Other features seen in the diffraction images are the very diffuse clouds of scattering. The location of the clouds, the bulk of which lie in the hk0 plane in the 3-3.5 Å resolution range, together with the fact that the disordered anti-codon loops lie perpendicular to the c axis, suggest that this component is the result of motion in the anti-codon region. Because this diffuse feature is not associated with Bragg positions and is smoothly varying, the disorder producing this scattering is some type of short-range correlated motion. The fact that these clouds are not continuous around the reciprocal origin suggests that the 6 Å coupling model used for the intra-molecular displacements in globular proteins may not be valid for tRNA.