Quaternary Organization of the 30S Ribosomal Subunit of Escherichia Coli.

is the density distribution in the image; x, y are coordinates relative to its center of gravity; and p, q are integers; p + q is the order of the moment. By combining these moments, certain invariants can be formed which may be used to describe the image with increasing accuracy as the order of moments included increases (14). Instead of the images themselves, the profiles formed by its moment-invariants d;, i = 1 ... n may therefore be used for the identification of projections in a micrograph of randomly oriented particles. To explore the feasibility of this approach, we have computed general (0,O) projections of the 30S ribosomal subunit previously reconstructed by Verschoor et al. (2), and rotated these in the image plane by various azimuths. The set of projections so created (Fig. 2 a) simulates the appearance of some of the particle views in the micrograph, assuming that the three-dimensional model is correct and provided that no deformations of the particle take place. The seven-moment invariants /l ... f7 associated with p + q = 3 that were given by Hawkes (I 1) were computed for each of the 40 (= 8 x 5 azimuths) projections. Because of quantization errors due to image rotation or due to sampling of differently oriented images, the calculated 0 values undergo fluctuations (see reference 12). At this stage of the investigation we would like to establish whether or not different projections of an asymmetric particle such as the ribosomal subunit give rise to recognizably different invariant profiles , and also whether or not the quantization errors would tend to obscure divisions among classes found. The invariant-profiles formed by /1 ... 07 were subjected to correspondence analysis (6, 7). What is apparent from the map of factor 1 vs. 3 (Fig. 2 b) is that the projections are recognized as different, irrespective of their in-plane orientation. (Similar grouping occurs on the map of factor 1 vs. 2 but with some ambiguities.) The model computation suggests that it may be possible, at least crudely, to identify molecular projections in noisy micrographs. Motif detection in quantum noise-limited electron micrographs by cross-correlation. Ultramicroscopy. Ribosome structure determined by electron micros-copy of Escherichia coli small subunits, large subunits, and monomeric ribosomes. The prokaryotic ribosome is composed of over fifty distinct proteins plus three different RNA molecules in two dissimilar subunits. Electron microscopy has been of great value in …