MUTANTS OF CHLOROPLAST COUPLING FACTOR REDUCTION IN ARABIDOPSIS

We have devised a two-step screening strategy for the selection of chl oroplast coupling factor reduction mutants from an M(2) population of Arabidopsis thaliana. The selection strategy relies on a lowered energ etic threshold for catalytic activation of the enzyme that has been sh own to accompany thioredoxin-mediated reduction of a cysteine bridge o n the gamma subunit of coupling factor. We selected first for plants t hat grew poorly under low irradiance but performed satisfactorily at h igh irradiance when the transmembrane electrochemical potential of hyd rogen ions is large and competent to maintain a high level of coupling factor activation without gamma subunit reduction. In the second step of the screen we monitored the flash-induced electrochromic change to select putative coupling factor reduction mutants from other sorts of mutations that shared the phenotype of poor growth and vigor when tra nsferred from high to low irradiance. Among the mutants selected, one appears incapable of reducing coupling factor; whereas another behaves as though coupling factor is at least partially reduced even in dark- adapted plants.