Isopycnic sucrose gradient analysis of the distribution of CAO. Membranes from C. reinhardtii cw15 cells that were grown 3 days in the dark at 28°C were layered on the top of 0.4 to 1.5 M linear sucrose gradients. After centrifugation as described in Methods, fractions were collected, and the proteins in each were precipitated with 10% (w/v) trichloroacetic acid and resolved by electrophoresis. A blot of the gel was probed first with antiserum against CAO and then with anti-LHCP IgG . (A), Graphic presentation of densitometric scans of immunoreactive bands shown in (B). (C), A diagram of the positions of visible amounts of membrane in the gradients. Chl (green), indicating the position of residual thylakoid membranes, was near the bottom, a yellow band was in the middle, and an orange, carotenoid-rich fraction at the top of the gradient.