Mutants of Chloroplast Coupling Factor Reduction in Arabidopsis.

We have devised a two-step screening strategy for the selection of chloroplast coupling factor reduction mutants from an M2 population of Arabidopsis thaliana. The selection strategy relies on a lowered energetic threshold for catalytic activation of the enzyme that has been shown to accompany thioredoxin-mediated reduction of a cysteine bridge on the [gamma] subunit of coupling factor. We selected first for plants that grew poorly under low irradiance but performed satisfactorily at high irradiance when the transmembrane electrochemical potential of hydrogen 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 transferred 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.