Suppressor mutations in chlamydomonas reveal a regulatory mechanism for flagellar function

Abstract

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, sup pf1 and sup pf2 , restore flagellar motility to either radial-spoke or central-pair mutants of different genes. The mutants sup pf3 and sup pf4 suppress flagellar paralysis associated only with mutants defective for the radial spokes. Analyses of the axonemal polypeptides of sup pf1 , sup pf3 and sup pf4 mutants indicate that the mutations restore flagellar activity to paralyzed radial-spoke or central-pair mutants by altering other components of the flagellar axoneme, sup pf1 shows an altered electrophoretic migration for a 325,000 molecular weight polypeptide known to be a subunit of an outer-arm dynein. sup pf3 and sup pf4 are missing different axonemal polypeptides with molecular weights of 60,000 (in the case of sup pf3 ), and 40,000 and 29,000 (in the case of sup pf4 ). Genetic evidence has been obtained indicating that the polypeptides affected in sup pf3 and sup pf4 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 generation of flagellar movements in the presence of radial-spoke or central-pair defects. Suppressor mutations release the inhibition. The molecular defects of sup pf1 , sup pf3 and sup pf4 provide evidence that the inhibitory mechanism can be interrupted at two different levels of axonemal function.