The bop2-1 mutation reveals radial asymmetry in the inner dynein arm region of Chlamydomonas reinhardtii.

Strains of Chlamydomonas reinhardtii with a mutant allele at the BOP2 locus swim slowly and have an abnormal flagellar waveform similar to previously identified strains with defects in the inner arm region. Double mutant strains with the bop2-1 allele and any of 17 different mutations that affect the dynein arm region swim more slowly than either parent, which suggests that the bop2-1 mutation does not affect solely the outer dynein arms, the I1 or ida4 inner dynein arms, or the dynein regulatory complex. Flagellar axonemes isolated from bop2-1 cells are missing a phosphorylated polypeptide of 152 kD. Electron microscopic analysis shows that bop2-1 axonemes are missing density in the inner dynein arm region. Surprisingly, two populations of images were observed in longitudinal sections of axonemes from the bop2-1 strain. In the 10 longitudinal axonemes examined, a portion of the dynein regulatory complex and a newly identified structure, the projection, are affected. In five of these 10 longitudinal axonemes examined, two lobes of the ida4 inner arm are also missing. By examining the cross-sectional images of wild-type and bop2-1 axonemes at each outer doublet position around the axoneme, we have determined that the bop2-1 mutation affects the assembly of inner arm region components in a doublet specific manner. Doublets 5, 6, and 8 have the most severe deficiency, doublet 9 has an intermediate phenotype, and doublets 2, 3, 4, and 7 have the least severe phenotype. The bop2-1 mutation provides the first evidence of radial asymmetry in the inner dynein arm region.

A genetic analysis of suppressors of the PF10 mutation in Chlamydomonas reinhardtii.

A mutation at the PF10 locus of the unicellular green alga Chlamydomonas reinhardtii leads to abnormal cell motility. The asymmetric form of the ciliary beat stroke characteristic of wild-type flagella is modified by this mutation to a nearly symmetric beat. We report here that this abnormal motility is a conditional phenotype that depends on light intensity. In the absence of light or under low light intensities, the motility is more severely impaired than at higher light intensities. By UV mutagenesis we obtained 11 intragenic and 70 extragenic strains that show reversion of the pf10 motility phenotype observed in low light. The intragenic events reverted the motility phenotype of the pf10 mutation completely. The extragenic events define at least seven suppressor loci; these map to linkage groups IV, VII, IX, XI, XII and XVII. Suppressor mutations at two of the seven loci (LIS1 and LIS2) require light for their suppressor activity. Forty-eight of the 70 extragenic suppressors were examined in heterozygous diploid cells; 47 of these mutants were recessive to the wild-type allele and one mutant (bop5-1) was dominant to the wild-type allele. Complementation analysis of the 47 recessive mutants showed unusual patterns. Most mutants within a recombinationally defined group failed to complement one another, although there were pairs that showed intra-allelic complementation. Additionally, some of the mutants at each recombinationally defined locus failed to complement mutants at other loci. They define dominant enhancers of one another.