The fine structure, protein composition, and roles in flagellar movement of specific axonemal components were studied in wild-type Chlamydomonas and paralyzed mutants pf-14, pf-15A, and pf-19. Electron microscope examination of the isolated axoneme of pf-14 showed that it lacks the radial spokes but is otherwise structurally normal. Comparison of isolated axonemes of wild type and pf-14 by sodium dodecyl sulfate-acrylamide gel electrophoresis indicated that the mutant is missing a protein of 118,000 mol wt; this protein is apparently a major component of the spokes. Pf-15A and pf-19 lack the central tubules and sheath; axonemes of these mutants are missing three high molecular weight proteins which are probably components of the central tubule-central sheath complex. Under conditions where wild-type axonemes reactivated, axonemes of the three mutants remained intact but did not form bends. However, mutant and wild-type axonemes underwent identical adenosine triphosphate-induced disintegration after treatment with trypsin; the dynein arms of the mutants are therefore capable of generating interdoublet shearing forces. These findings indicated that both the radial spokes and the central tubule-central sheath complex are essential for conversion of interdoublet sliding into axonemal bending. Moreover, because axonemes of pf-14 remained intact under reactivating conditions, the nexin links alone are sufficient to limit the amount of interdoublet sliding that occurs. The axial periodicities of the central sheath, dynein arms, radial spokes, and nexin links of Chlamydomonas were determined by electron microscopy using the lattice-spacing of crystalline catalase as an internal standard. Some new ultrastructural details of the components are described.

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