PARENT SESSION
Posters P7B Evolution of photosynthesis. Abstracts (579-591)

Genome-wide analysis of the PSII reaction center D1 protein: Is Gloeobacter violaceous the missing link in evolution of oxygenic photosynthesis? Tuan Nguyen^*,1, Rogier van Willigen¹, G. Charles Dismukes¹, ¹ Dept. of Chemistry, Princeton, NJ, USA

ABSTRACT- The D1/D2 subunits comprise the reaction center core of the O2-evolving PS-II complex. D1 provides most of the ligands to the water-oxidizing complex (Mn4Ca1OxCly). This study examines the lack of diversity of D1 amino acid sequences (> 70% identity) in more than one hundred oxygenic phototrophs. Both the ligands to the water-oxidizing complex and the inorganic cofactors are extremely conserved, which expresses very strong restrictions for water splitting function. Diversity among organisms occurs at the genome level in terms of the number (1-5) and location of the psbA genes. We have found no conservation of the order of neighboring genes around the psbA loci, nor is prokaryotic psbA gene co-transcribed in operon with other PSII-related genes. The sole possible exception is in the recently sequenced Gloeobacter violaceous genome: one of the 5 psbA copies is clustered with the known single copy psbDC operon. We postulate that psbADC is an operon in Gloeobacter and could represent an ancestoral genomic arrangement, resembling anoxygenic photoautotrophs where reaction center subunits (L/M) are co-transcribed. The ribosome binding site of the psbA gene in the psbADC cluster scores as the strongest among the other 4 copies and thus this copy would be highly expressed. We shall present experimental data on the expression of psbA genes and whether a functional psbADC operon exists in Gloeobacter. Together with biophysical data showing highly inefficient water splitting chemistry, these features suggest that Gloeobacter may be a closest contemporary example of a "missing link" in the evolution of oxygenic photosynthesis.

KEY WORDS: photosynthesis, evolution, D1 protein, gloeobacter