MUTATIONAL ANALYSIS OF THE PHOTOTRANSDUCTION PATHWAY OF CHLAMYDOMONAS-REINHARDTII

Chlamydomonas has two photobehavioral responses, phototaxis and photos hock. Rhodopsin is the photoreceptor for these responses and the signa l transduction process involves transmembrane Ca2+ fluxes. This causes transient changes in flagellar beating, ultimately resulting in photo taxis or photoshock, To identify components that make up this signal t ransduction pathway, we generated nonphototactic strains by insertiona l mutagenesis. Seven new phototaxis genes were identified (ptx2 - ptx8 ); alleles of six of these are tagged by the transforming DNA and ther efore should be easily cloned. To order the mutants in the pathway, we characterized them electrophysiologically, behaviorally, and structur ally. ptx5, ptx6, and ptx7 have normal light-induced photoreceptor cur rents (PRC) and flagellar currents (FC) but their pattern of swimming does not change in the normal manner when the intraflagellar Ca2+ conc entration is decreased, suggesting that they have defects in the abili ty of their axonemes to respond to changes in Ca2+ concentration. ptx2 and ptx8 lack the FC but have normal PRCs, suggesting that they are d efective in the flagellar Ca2+ channel or some factor that regulates i t. ptx4 mutants have multiple eyespots. ptx3 mutants are defective in a component essential for phototaxis but bypassed during photoshock; t his component appears to be located downstream of the PRC but upstream of the axoneme.