K-125. Optimizing Photosynthetic Light Harvesting Capacity: Molecular Evolutionary and Functional Genomic Analyses of the Chlorophyll-Binding Antenna in Prochlorococcus

M. E. Ramsey, C. S. Ting;
Williams Coll., Williamstown, MA.

The photosynthetic marine prokaryote Prochlorococcus is broadly distributed throughout the water column of subtropical and tropical oceans and is an important contributor to primary production. Unlike most cyanobacteria, Prochlorococcus uses a chlorophyll a/b light harvesting complex encoded by the pcb genes. The Pcb proteins are part of a larger family that includes IsiA (an iron stress-inducible protein) and CP43 (a Photosystem II core antenna protein), and as many as eight pcb genes can be found in some Prochlorococcus strains. With the availability of over 50 pcb sequences from cultured strains and environmental samples, we are investigating the molecular evolution of these sequences in addition to employing qRT-PCR to characterize pcb expression after stress exposure. We hypothesize that differential gene regulation significantly affects both environmental stress response and the ability of these strains to thrive under a variety of irradiance conditions. Comparative analysis of functional domains (six transmembrane helices, five intervening loop regions) indicates several highly conserved insertions in the loop regions of the cytoplasmic face that are absent in CP43 and IsiA. These conserved insertions likely play a significant role in the assembly and/or structure of the Pcb complexes, which associate with the Photosystem I/II reaction centers. Molecular evolutionary analyses indicate that all sequences are under negative selection (Ka/Ks < 1), although nonsynonymous changes are elevated among some pcb sequences in the SS120 strain, which contains eight pcb genes. Relative rate tests indicate a more rapid accumulation of mutations in SS120, a strain that thrives at very low irradiance levels, suggesting that sequence multiplication and divergence may be conferring an adaptive advantage at low light. Preliminary expression data support this hypothesis and suggest that pcb expression is coupled to variation in irradiance level. Integration of comparative and functional genomics provides a powerful approach for advancing our understanding of the evolution and function of these key photosynthetic apparatus proteins.