Expression of a ketolase gene mediates the synthesis of canthaxanthin in synechococcus leading to tolerance against photoinhibition, pigment degradation and UV-B sensitivity of photosynthesis

The potential of ketocarotenoids to protect the photosynthetic apparatus fr om damage caused by excess light and UV-B radiation was assessed. Therefore , the cyanobacterium Synechococcus was transformed with a foreign beta -car otene ketolase gene under a strong promoter leading to the accumulation of canthaxanthin, This diketo carotenoid is absent in the original strain. Mos t of the newly formed canthaxanthin was located in the thylakoid membranes. The endogenous beta -carotene hydroxylase was unable to interact with the ketolase, Therefore, only traces of astaxanthin were found. The transforman t was treated with strong light (500 or 1200 mu mol m(-2) s(-1))and with UV -B radiation. In contrast to a nontransformed strain the overall photosynth esis, measured as oxygen evolution, was protected from inhibition by light of 500 mu mol m (2) s(-1) and UV-B radiation of 6.8 W m(-2). Furthermore, d egradation in the light of chlorophyll and carotenoids at an irradiance of 1200 mu mol m(-2) s(-1), which was substantial in the nontransformed contro l, was prevented. These results indicate that in situ canthaxanthin, which is formed at the expense of zeaxanthin and replaces this hydroxy carotenoid within the photosynthetic apparatus, is a better protectant against solar radiation, These findings are discussed on the basis of the in vitro proper ties such as inactivating peroxyl radicals, quenching of singlet oxygen and oxidation stability of these different carotenoid structures.