The evolution of photosystem (PS) I was probably initiated by the formation of a
homodimeric reaction center similar to the one currently present in green
bacteria. Gene duplication has generated a heterodimeric reaction center that
subsequently evolved to the PSI present in cyanobacteria, algae and plant
chloroplasts. During the evolution of PSI several attempts to maximize the
efficiency of light harvesting took place in the various organisms. In the
Chlorobiaceae, chlorosomes and FMO were added to the homodimeric reaction
center. In cyanobacteria phycobilisomes and CP43' evolved to cope with the light
limitations and stress conditions. The plant PSI utilizes a modular arrangement
of membrane light-harvesting proteins (LHCI). We obtained structural information
from the two ends of the evolutionary spectrum. Novel features in the structure
of Chlorobium tepidum FMO are reported in this communication. Our structure of
plant PSI reveals that the addition of subunit G provided the template for LHCI
binding, and the addition of subunit H prevented the possibility of trimer
formation and provided a binding site for LHCII and the onset of energy
spillover from PSII to PSI.
