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 single-particle analysis of MIT 9313 and showed that eight Pcb proteins associate with photosystem II (PSII)—four on each side of the dimer. It is only when MIT 9313 cells are grown in the absence of iron that the 18-mer Pcb–PSI supercomplex is observed. Expression analysis of the pcbA and pcbB genes revealed that in medium supplemented with iron, the PcbA protein accumulated. However, cells grown without iron initiated pcbB expression combined with downregulation of pcbA expression and of the PSI and PSII reaction center proteins psaA and psbA. Protein sequence analysis of the Pcb–PSI supercomplex from these cells confirmed that the Pcb protein present is PcbB. Despite a detectable decrease in pcbA expression, the PcbA protein levels remained relatively high with the protein still complexed to PSII. Further analysis confirmed that the PSI:PSII ratio remained unchanged in the presence or absence of iron. Bibby et al. propose that the PcbA protein targets to PSII and that PcbB targets to PSI, increasing the light-harvesting capability of the respective photosystem.

"Here we show that the moderate low-light adapted strain Prochlorococcus sp. MIT 9313 has one iron-stress-induced pcb gene encoding an antenna protein serving PSI—comparable to isiA genes from cyanobacteria—and a constitutively expressed pcb gene encoding a PSII antenna protein. By comparison, the very-low-light adapted strain SS120 has seven pcb genes encoding constitutive PSI and PSII antennae, plus one PSI iron-regulated pcb gene, whereas the high-light–adapted strain MED4 has only a constitutive PSII antenna. Thus, it seems that the adaptation of Prochlorococcus to low light environments has triggered a multiplication and specialization of Pcb proteins comparable to that found for Cab proteins in plants and green algae," conclude the authors.