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Carotenoid biosynthesis is up-regulated by strong light in the cyanobacterium Synechococcus . By blocking off the pathway at the level of phytoene conversion, lightdependent accumulation of phytoene was observed. Realtime PCR studies demonstrated that four genes of the carotenogenic pathway are under transcriptional control. These were the genes encoding phytoene synthase, phytoene desaturase, z -carotene desaturase and b -carotene hydroxylase. The transcript of the first three follow a similar kinetics, whereas the transcript of b -carotene hydroxylase increased much faster upon transfer to high light. Promoter activities were determined with transcriptional fusions to chloramphenicol acyltransferase as reporter enzyme. The activity of the promoter of the phytoene desaturase/synthase operon was higher under strong light.

Antioxidants, Glycolate Oxidase, PhotoinactivationGlycolate oxidase that was partially purified from pea leaves was inactivated in vitro by blue light in the presence of FMN. Inactivation was greatly retarded in the absence of 0 2. Under aerobic conditions H20 2 was formed. The presence of catalase, GSH or dithiothreitol protected glycolate oxidase against photoinactivation. Less efficient protection was provided by ascorbate, histidine, tryptophan or EDTA. The presence of superoxide dismutase or of hydroxyl radical scavengers had no, or only minor, effects. Glutathione suppressed H20 2 accumulation and was oxidized in the presence of glycolate oxidase in blue light. Glycolate oxidase was also inactivated in the presence of a superoxide-generating system or by H20 2 in darkness. In intact leaves photoinactivation of glycolate oxidase was not observed. How ever, when catalase was inactivated by the application of 3-amino-l,2,4-triazole or depleted by prolonged exposure to cycloheximide a strong photoinactivation of glycolate oxidase was also seen in leaves. In vivo blue and red light were similarly effective. Furthermore, glycolate oxidase was photoinactivated in leaves when the endogenous GSH was depleted by the application of buthionine sulfoximine. Both catalase and antioxidants, in particular GSH, appear to be essential for the protection of glycolate oxidase in the peroxisomes in vivo.