Prochlorococcus is the most abundant unicellular photosynthetic organism, is found throughout the eutrophic zone, and has a number of strains adapted to a range of light conditions. Its chlorophyll-binding, light-harvesting proteins are encoded by the pcb genes. The very low-light–adapted strain SS120 maintains eight Pcb proteins (PcbA-H), the moderate low-light–adapted strain MIT 9313 uses PcbA and PcbB, and the high-light–dependent strain MED4 has just a single pcb gene—pcbA. Under normal conditions, SS120 photosystem I (PSI) is surrounded by 18 Pcb subunits—analogous to the 18-mer IsiA–PSI supercomplex formed in cyanobacteria following iron deprivation. In the August 28 Nature, Thomas S. Bibby and colleagues at Imperial College London examined gene expression and performed structural analysis of MIT 9313 Pcb proteins. They observed that this strain has specific Pcb antenna proteins for each of the photosystems (Nature 424:1051-1054, August 28, 2003).

Bibby et al. performed electron microscopy and...

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