Crystallographic structure of the octamer histone core of the nucleosome.

should be compared with figure 2a in (2).These maps are "before" and "after" solvent flattening respectively, and clearly demonstrate that our noise filtering procedure did not introduce artifacts into the map. 4) Klug et al., seem to have overlooked the information presented (2, p. 547) and concluded that we have incorrectly chosen the protein boundaries. On the contrary, 95 percent of the boundaries of the octamer are unambiguously delineated by large solvent regions between molecules [Fig. 1 and figure 2 in (2)] and the constraints imposed by the crystallographic symmetry elements (2, p. 547). Five helices, not two, were mentioned in our paper (2), and these plus several smaller helices make up about 50 percent of the protein mass, consistent with circular dichroism and Raman spectroscopy (16). Furthermore, we have reported that the chains of H2A and H2B have been traced nearly from end to end, and sufficient segments of characteristic amino acid sequences have been identified in the map to allow the assignment of the polypeptides. The quality of the map itself proves the validity of our procedures. The accompanying stereo pair (Fig. 2) is a replot of the alpha-helical region we showed before [figure 1 in (2)], contoured at a lower level and thus illustrating some side chains. We believe that the resolution of the causes for the differences between the two structures will come about through further experimentation rather than rhetoric and argumentation. The results of our ongoing efforts in fitting the amino acids to the electron density map should shed some light on the resolution of the differences between the two structures. E. N. MOUDRIANAKIS Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218 W. E. LOVE Department of Biophysics, Johns Hopkins University B. C. WANG Biocrystallography Laboratory, V.A. Medical Center, Pittsburgh, Pennsylvania 15240, and Department of Crystallography, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 N. G. XUONG Department of Physics, University of California at San Diego, La Jolla 92093 R. W. BURLINGAME Department of Biology, Johns Hopkins University