Analysis of the movement of Chlamydomonas flagella: The function of the radial-spoke system is revealed by comparison of wild-type and mutant flagella

Abstract

The mutation uni-1 gives rise to uniflagellate Chlamydomonas cells which rotate around a fixed point in the microscope field, so that the flagellar bending pattern can be photographed easily. This has allowed us to make a detailed analysis of the wild-type flagellar bending pattern and the bending patterns of flagella on several mutant strains. Cells containing uni-1, and recombinants of uni-1 with the suppressor mutations, sup(pf)-1 and sup(pf)-3, show the typical asymmetric bending pattern associated with forward swimming in Chlamydomonas, although sup(pf)-1 flagella have about one-half the normal beat frequency, apparently as the result of defective function of the outer dynein arms. The pf-17 mutation has been shown to produce nonmotile flagella in which radial spoke heads and five characteristic axonemal polypeptides are missing. Recombinants containing pf-17 and either sup(pf)-1 or sup(pf)-3 have motile flagella, but still lack radial-spoke heads and the associated polypeptides. The flagellar bending pattern of these recombinants lacking radial-spoke heads is a nearly symmetric, large amplitude pattern which is quite unlike the wild-type pattern. However, the presence of an intact radial-spoke system is not required to convert active sliding into bending and is not required for bend initiation and bend propagation, since all of these processes are active in the sup(pf)pf-17 recombinants. The function of the radial-spoke system appears to be to convert the symmetric bending pattern displayed by these recombinants into the asymmetric bending pattern required for efficient swimming, by inhibiting the development of reverse bends during the recovery phase of the bending cycle.