Negative stain EM shows actin assembly at the contractomere. Negative-stain electron microscopy of an actin assembly reaction (see Methods). Upper panel shows a membrane sheet containing contractomeres (circled) densely decorated with actin filaments. Lower panel shows an enlarge contractomere (circled) with attached actin bundles (arrows). An enlarged contractomere wrapped in actin is shown in Fig. S11 where the top panel is shown again illustrating the area of enlargement. Scale bars are 2 μm for the upper panel and 200 nm for the lower panel.