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Notes: Summary Negatively stained paracrystals of reconstituted thin filaments decorated with myosin subfragment 1 (S1), at high calcium concentrations (≥10−5 m), exhibitpgg plane group symmetry with component filaments having 28 subunits in 13 turns of the actin genetic helix. Isolated S1 decorated F-actin filaments trapped in a stain film were also observed to form spontaneously paracrystals withpgg plane group symmetry. We conclude that a favourable S1-S1 interaction must exist in order to stabilize these structures. Three-dimensional helical reconstructions, calculated from these paracrystals show S1 to be curved, 12 to 14 nm long and tilted with respect to the helical axis, in broad agreement with previous reconstructions calculated from isolated particles. Reconstructions of S1 and HMM decorated filaments that resolve actin show a principal myosin binding site located on the side of the actin subunit reported by Taylor & Amos [J. molec. Biol. 147, 297–324 (1981)] and a possible small interaction on the opposite side. The appearance, symmetry and helical reconstructions of iolated F-actin filaments decorated with heavy meromyosin (HMM) were similar to those of S1 decorated filaments, except at high radii where extra mass was observed. This probably arose from the connection between the two heads of HMM bound to the same long-pitch strand of actin. In contrast to most studies on thin filaments, which use reconstituted filaments, we present data on natural I-segments of muscle homogenates. Individual filaments exhibited actin helical symmetry which on reconstruction gave a two-domain motif oriented constitently with its long axis approximately perpendicular to the helical axis, but inclined towards the 5.9 nm genetic helix. Our original interpretation of these maps [Seymour & O'Brien,Nature, Lond. 283, 680–2 (1980)] depended upon reconstructions from F-actin paracrystals, which suggested actin was rather symmetrical in shape. New data from tw- and three-dimensional crystal studies and reconstructions of actin-tropomyosin filaments show that actin is rather elongated and consists of two domains. These results indicate that actin contributes towards both domains of our I-segment motif and are consistent with the monomer long axis lying approximately perpendicular to the helical axis. Although tropomyosin is not resolved, comparison of the actin-tropomyosin and I-segment reconstructions suggests that tropomyosin is strongly merged with the actin domain at a lower radius from the helical axis and that the domain at higher radius arises solely from actin.