PARENT SESSION

Symposium S2D Marine photosynthesis and production
Monday August 30th, 2004 2:40 PM-4:40 PM Room 510B
Chair: John Raven
Co-Chair: Doug Campbell

Functional and comparative genome analysis of photosynthesis in Prochlorococcus. Wolfgang Hess^{*,1, 6}, Claudia Steglich², Frederic Partensky³, Alexis Dufresne³, Michael Galperin⁴, Anne Rediger⁵, ¹ University Freiburg, Freiburg, Germany⁶ Humboldt-University Berlin, Berlin, Germany² Massachusetts Institute of Technology, Cambridge, MA, U.S.³ Station Biologique Roscoff CNRS, Roscoff, France^{4 5} National Center for Biotechnology Information, NLM, National Institutes of Health, Bethesda, MD, USA

ABSTRACT- Recently, total genome sequences became available for four closely related marine cyanobacteria, Prochlorococcus marinus SS120, MED4, MIT9313, and Synechococcus WH8102 [1-4]. Prochlorococcus is an interesting organism because instead of phycobilisomes it uses chlorophyll (Chl) b and a Chl a/b-binding protein for light harvesting and since it has a remarkable pigment composition with high amounts of divinyl-Chl b, both alpha and beta carotenoids as well as phycoerythrin as the sole phycobiliprotein. The genomes of these cyanobacteria are remarkable small and compact. With a length of only 1.6 and 1.8 million base pairs, Prochlorococcus MED4 and SS120 represent the minimal genome for a free-living photoautotroph. Prochlorococcus exists in the form of distinct ecotypes which have been adapted genetically to different environmental conditions (high or low irradiance conditions) and dominate either the surface or deeper waters. During genome analysis it turned out that these genomes differ by hundreds of genes (5). Thus, they provide an excellent comparative dataset for the identification of the core gene set for photosynthesis, of distinct evolutionary adaptations to particular light conditions and of gene and protein functions that had remained unidentified so far. Genes for photosystem II proteins that are frequently found in several copies in cyanobacteria are single copy genes (psbA, psbD) or have been eliminated altogether (psbU, psbV). In contrast, in the ultra-low light adapted strain SS120 a gene family of eight different light harvesting proteins evolved to cope with these particular conditions. Yet, there are also two forms of a functional phycoerythrin in Prochlorococcus, which are characteristic for either the low or the high light-adapted ecotype. A novel type of lycopene cyclase is involved in the formation of carotenoids in these cyanobacteria. REFERENCES: [1] Photosynth. Res. (2001) 70, 53-72; [2] Proc. Natl. Acad. Sci. USA (2003) 100, 10020-10025; [3] Nature (2003) 424, 1042-1047; [4] Nature (2003) 424, 1037-1042; [5] Curr. Op. Biotechnol. (2004) 15, 191-198.

KEY WORDS: genomics, marine cyanobacteria, photosynthesis, light harvesting proteins