Suppressor mutations in Chlamydomonas reveal a regulatory mechanism for Flagellar function.

Reversion analysis of flagellar-motility mutants of Chlamydomonas reinhardtii yields an unusual class of intergenic suppressor mutations that restore flagellar activity to paralyzed radial-spoke or central-pair mutants without altering the structural or molecular defects associated with the original mutations. Four suppressors representing independent genetic loci were studied in detail. Two of the mutations, suppf1 and suppf2, restore flagellar motility to either radial-spoke or central-pair mutants of different genes. The mutants suppf3 and suppf 4 suppress flagellar paralysis associated only with mutants defective for the radial spokes. Analyses of the axonemal polypeptides of suppf1, suppf3 and suppf4 mutants indicate that the mutations restore flagellar activity to paralyzed radial-spoke or central-pair mutants by altering other components of the flagellar axoneme. suppf1 shows an altered electrophoretic migration for a 325,000 molecular weight polypeptide known to be a subunit of an outer-arm dynein. suppf3 and suppf4 are missing different axonemal polypeptides with molecular weights of 60,000 (in the case of suppf3), and 40,000 and 29,000 (in the case of suppf4). Genetic evidence has been obtained indicating that the polypeptides affected in suppf3 and suppf4 are components of a newly identified functional and/or structural compartment of the flagellar axoneme. The suppressor mutations described here reveal the operation of a control mechanism that inhibits the operations of flagellar movements in the presence of radial-spoke or central-pair defects. Suppressor mutations release the inhibition. The molecular defects of suppf1, suppf3 and suppf4 provide evidence that the inhibitory mechanism can be interrupted at two different levels of axonemal function.