A Role of the QB Binding Protein in the Mechanism of Cyanobacterial Adaptation to Light Intensity?*

G row th o f the unicellular b lue-green alga Anacystis nidulans in m edia containing sublethal concentrations o f D C M U -typ e inhibitors o f p hotosynthetic electron transport in strong w hite light gave rise to shade type appearance in this organism , as characterized by an increased ratio o f phycocyanin to chlorophyll and reduced ratios, b oth , o f carotenoids to chlorophyll and o f total chlorophyll to P700. Shade type in Anacystis w as caused n either by phenolic inhibitors tested nor by those known to bind to the cytochrom e ^ /c o m p le x . Surprisingly enough , the m olar ratio o f phycocyanin to chlorophyll in artificially shade adapted Anacystis, grown in strong w hite light in the presence o f 1 0 -6 m atrazine, w as found to increase with tem perature for a given light intensity and with light intensity for a g iven tem perature. M utants o f Anacystis with a reduced binding capacity for D C M U -typ e herbicides due to an am ino acid exchange in the 32 k D a Q B-binding p o lyp ep tid e , also called D l protein , w ere o b ­ served to show shade type appearance in strong light, to respond very little to changes in light intensity and to show a reduced capability to further change their appearance to shade type by binding o f com petitors o f Q B to the 32 kD a polyp ep tid e. In Anacystis a concentration o f atrazine (1 0 “7 m ) , ten tim es low er than the one causing the highest rate o f shade adaptation (10~6 m ) , w as show n to induce an optim um in cell density, which in turn resulted in an optim um in ligh t-dependent 0 2 evo lu tion . B oth factors together might be responsible for the so-called greening effect observed in higher plants treated with sublethal concentrations o f D C M U -typ e inhibitors o f photosyn th etic electron transport.