Review Genomic analysis of mutants affecting xanthophyll biosynthesis and regulation of photosynthetic light harvesting in Chlamydomonas reinhardtii M. Anwaruzzaman 1 , Brian L. Chin 1 , Xiao-Ping Li 1 , Martin Lohr 2 , Diego A. Martinez 3 & Krishna K. Niyogi 1,* 1 Department of Plant and Microbial Biology, 111 Koshland Hall, University of California, Berkeley, CA 94720-3102, USA; 2 Johannes Gutenberg-Universita ¨t, Institut fu ¨r Allgemeine Botanik, Mu ¨llerweg 6, 55128 Mainz, Germany; 3 US Department of Energy Joint Genome Institute, 2800 Mitchell Drive, Walnut Creek, CA 94598, USA; *Author for correspondence (e-mail: niyogi@nature.berkeley.edu; fax: +1-510-642-4995) Received 13 January 2004; accepted in revised form 17 June 2004 Key words: lutein, lycopene e-cyclase, non-photochemical quenching, PsbS, violaxanthin de-epoxidase, xanthophyll cycle, zeaxanthin Abstract When the absorption of light energy exceeds the capacity for its utilization in photosynthesis, regulation of light harvesting is critical in order for photosynthetic organisms to minimize photo-oxidative damage. Thermal dissipation of excess absorbed light energy, measured as non-photochemical quenching (NPQ) of chlorophyll fluorescence, is induced rapidly in response to excess light conditions, and it is known that xanthophylls such as zeaxanthin and lutein, the transthylakoid pH gradient, and the PsbS protein are involved in this mechanism. Although mutants affecting NPQ and the biosynthesis of zeaxanthin and lutein were originally isolated and characterized at the physiological level in the unicellular green alga Chla- mydomonas reinhardtii, the molecular basis of several of these mutants, such as npq1 and lor1, has not been determined previously. The recent sequencing of the C. reinhardtii nuclear genome has facilitated the search for C. reinhardtii homologs of plant genes involved in xanthophyll biosynthesis and regulation of light harvesting. Here we report the identification of C. reinhardtii genes encoding PsbS and lycopene e-cyclase, and we show that the lor1 mutation, which affects lutein synthesis, is located within the lycopene e-cyclase gene. In contrast, no homolog of the plant violaxanthin de-epoxidase (VDE) gene was found. Molecular markers were used to map the npq1 mutation, which affects VDE activity, as a first step toward the map- based cloning of the NPQ1 gene. Abbreviations: Chl – chlorophyll; EST – expressed sequence tag; LCYE – lycopene e-cyclase; LHC – light- harvesting complex; NPQ – non-photochemical quenching; PS – Photosystem; qE – DpH-dependent component of non-photochemical quenching; VDE – violaxanthin de-epoxidase; ZEP – zeaxanthin epoxidase Introduction Light is vital for the growth and survival of all photoautotrophic organisms such as green algae and plants. However, absorption of light in excess of the capacity for light energy utilization in photosynthetic CO 2 fixation can lead to increased production of damaging reactive oxygen species as byproducts of photosynthesis (reviewed in Asada 1994; Niyogi 1999). Because an imbalance between light absorption and utilization occurs often in natural, fluctuating light environments, regulation Photosynthesis Research 82: 265–276, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 265