Isolation and characterization of novel Chlamydomonas mutants that displayphototaxis but not photophobic response

The unicellular green alga Chlamydomonas displays two distinct kinds of beh avioral response to light: phototaxis, in which cells swim toward or away f rom the light source under constant illumination; and photophobic responses (also called stop responses or photoshock responses), in which cells trans iently convert their flagellar waveform and swim backward upon sudden incre ase in light intensity. It has been suggested that the two responses partly share a common signal transduction pathway, but exactly how the different responses are produced has not been established. In this study, to help und erstand the molecular and cellular mechanisms that bring about the photopho bic response, we isolated novel mutants (ppr1, ppr2, ppr3, and ppr4) that d o not show the photophobic response. Importantly, these mutants retain the ability to display phototaxis, with almost the same sensitivities as in the wild type cell. Demembranated and reactivated flagellar axonemes of the pp r mutants were found to convert the bending patterns depending on the Ca2concentration, indicating that the axonemal mechanism for waveform conversi on required for the photophobic response was unaffected by the mutations. I n addition, measurements of electric currents in cell suspensions showed th at these mutants generate normal photoreceptor currents (:PRC) upon photost imulation, suggesting that they retain the normal activity of photoreceptio n and the ionic channels that produce PRCs. However, the all-or-none flagel lar current (FC), a Ca2+ current generated by PRC-induced depolarization of flagellar membrane, was absent or seriously impaired in the mutants. These findings clearly indicate that the all-or-none FC is necessary for the pho tophobic response but not for phototaxis. The isolation of the four genetic ally independent ppr mutants suggests that the generation of the FC is base d on multiple components that are not used in the mechanism for phototaxis, and implies that the Chlamydomonas flagellar membrane possesses a voltage- dependent Ca2+-channel specifically used for generation of photophobic resp onses. (C) 1998 Wiley-Liss, Inc.