FLAGELLAR PHOTORESPONSES OF PTX1, A NONPHOTOTACTIC MUTANT OF CHLAMYDOMONAS

Phototaxis in wild-type Chlamydomonas cells is probably the result of four single photoresponses in the flagella, inverse in the cis (near t he eyespot) and in the trans flagellum and also inverse by step-up (in crease of) and step-down (decrease of) light stimulation as experience d by the eyespot during rotation of the cell [Ruffer and Nultsch, 1991 : Cell Motil. Cytoskeleton 18:269-278]. Two inverse sets of the four r esponses are supposed to be the cause for positive and negative photot axis. The relevant flagellar responses consist of shifts of the front amplitude of the breaststroke beats. As single flagellar responses can not be called ''phototactic'' they are termed ''breaststroke flagellar photoresponses.'' The mutant strain ptx1 is defective in phototaxis b ut displays photoshocks [Horst and Witman, 1993: J. Cell Biol. 120:733 -741]. Analysis of flagellar beat patterns in high-speed records of pt x1 cells held on micropipettes shows that breaststroke flagellar photo responses exist in this mutant in spite of the loss of phototaxis. It is the cis/trans differentiation that is lost in ptx1: both flagella a lways respond in the same way and not inversely as in wild-type cells. Equal shifts of beat amplitude cannot cause a turn of the cell, which explains why phototaxis is not seen in ptx1 and supports the model su ggested for positive and negative phototactic steering. In wild-type c ells front amplitude changes are connected with beat period changes, w hich also occur in ptx1 cells and suggest that both Flagella respond l ike wild-type trans flagella. Divergencies in the shock response of pt x1 cells, beat period reduction, and coordination changes may support the notion that cis flagellar specialization is lost and that ptx1 pos sesses, so to speak, two trans and no cia flagellum. Therefore, the mu tant strain ptx1 might be useful for studying molecular and functional peculiarities of the two flagella. (C) 1997 Wiley-Liss, Inc.