cmu1-1 is a new mutation of Chlamydomonas reinhardtii that causes a change in cell shape due to an alteration of cytoplasmic microtubule organization. cmu1 mutant cells were first identified based on their altered cell shape. Unlike wild-type cells, which are ellipsoid, cmu1 cells tend to be either round or egg-shaped with the flagella extending from the narrow end of the cell. Electron microscopic comparison of mutant and wild-type cells indicated that microtubule distribution was altered in the mutant cells. Immunofluorescence microscopy using anti-beta-tubulin antibodies revealed that, in wild-type cells, microtubules arise from the anterior end of the cell in the region of the basal bodies, pass posteriorly subjacent to the plasma membrane, and terminate near the posterior end of the cell. In mutant cells, the microtubules also arise from the basal body region but then become disarrayed. They frequently curl back anteriorly or wrap around the equator of the cell; some microtubules also extend completely to the posterior end of the cell, then turn back toward the anterior end. No changes in the basal body region were detected by electron microscopy. Some cmu1 cells had multiple nuclei or an aberrant number of flagella, both of which may be due to defects in cell division, a process dependent upon microtubules. Thus, cmu1-1, which was generated by insertional mutagenesis and is tagged, appears to encode a protein that plays an essential role in the spatial organization of cytoplasmic microtubules involved in both interphase and mitotic functions.

A new mutant strain of Chlamydomonas, ptx1, has been identified which is defective in phototaxis. This strain swims with a rate and straightness of path comparable with that of wild-type cells, and retains the photoshock response. Thus, the mutation does not cause any gross defects in swimming ability or photoreception, and appears to be specific for phototaxis. Calcium is required for phototaxis in wild-type cells, and causes a concentration-dependent shift in flagellar dominance in reactivated, demembranated cell models. ptx1-reactivated models are defective in this calcium-dependent shift in flagellar dominance. This indicates that the mutation affects one or more components of the calcium-dependent axonemal regulatory system, and that this system mediates phototaxis. The reduction or absence of two 75-kD axonemal proteins correlates with the nonphototactic phenotype. Axonemal fractionation studies, and analysis of axonemes from mutant strains with known structural defects, failed to reveal the structural localization of the 75-kD proteins within the axoneme. The proteins are not components of the outer dynein arms, two of the three types of inner dynein arms, the radial spokes, or the central pair complex. Because changes in flagellar motility ultimately require the regulation of dynein activity, cell models from mutant strains defective in specific dynein arms were reactivated at various calcium concentrations. Mutants lacking the outer arms, or the I1 or I2 inner dynein arms, retain the wild-type calcium-dependent shift in flagellar dominance. Therefore, none of these arms are the sole mediators of phototaxis.