Miniature- and Multiple-Eyespot Loci in Chlamydomonas reinhardtii Define New Modulators of Eyespot Photoreception and Assembly

摘要

pThe photosensory eyespot of the green alga iChlamydomonas reinhardtii/i is a model system for the study of organelle biogenesis and placement. Eyespot assembly and positioning are governed by several genetic loci that have been identified in forward genetic screens for phototaxis-defective mutants. These include the previously described miniature-eyespot mutant imin1/i, the multiple-eyespot mutant imlt1/i, the eyeless mutants ieye2/i and ieye3/i, and two previously uncharacterized eyespot mutants, imin2/i and imlt2/i. In this study, effects of miniature- and multiple-eyespot mutations and their combinations on the localization and expression levels of the rhodopsin photoreceptor channelrhodopsin-1 (ChR1) and the localization of the eyespot-assembly proteins EYE2 and EYE3 were examined. imin2/i mutants assemble a properly organized, albeit nonfunctional, eyespot that is slightly smaller than wild-type; however, combination of the imin2/i and imlt1/i mutations resulted in drastic reduction of photoreceptor levels. Both stationary-phase imlt1/i and imlt2/i cells have supernumerary, mislocalized eyespots that exhibit partial or total dissociation of the eyespot layers. In these mutant strains, photoreceptor patches in the plasma membrane were never associated with pigment granule arrays in the chloroplast stroma unless EYE2 was present in the intervening envelope. The data suggest that MIN2 is required for the photoreceptive ability of the eyespot and that MLT2 plays a major role in regulating eyespot number, placement, and integrity./p