| Parkin-co-regulated gene (PACRG) is a human gene whose transcription is co-regulated with a Parkinson's disease-related gene. Recently, a Trypanosome homologue of the PACRG gene product has been suggested to act to maintain stability of flagellar outer doublet microtubules (Dawe et al., 2005), and a Chlamydomonas homologue has been localized to outer doublets by immuno-electron microscopy (Ikeda et al., 2007). In this study, we isolated a Chlamydomonas mutant that completely lacks the PACRG protein by screening mutants with abnormal motility phenotypes. The mutation mapped to the pf12 locus. Both this mutant (pf12-2) and pf12 were found to have single nucleotide substitutions in the PACRG gene that result in premature stop codons. The mutant phenotype was rescued by introduction of either the PACRG gene or its cDNA cloned into the PsaD expression cassette. The flagella of the mutants have almost normal length, but beat with an abnormal, almost symmetrical pattern, somewhat like those of the backward-swimming mutants mbo1-3 (Segal et al, 1984). Like mbo1-3, pf12 lacks beak structures within the B-tubule of #5 and #6 doublets. However, PACRG does not appear to be a major component of the beaks, since mbo1-3 axonemes retain normal amounts of PACRG. Rather, PACRG may function as a scaffold for the assembly of substructures in the outer doublets that are necessary for the proper function but not assembly of the axoneme.
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