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Prochlorococcus, the most abundant genus of photosynthetic organisms1, owes its remarkably large depth distribution in the oceans to the occurrence of distinct genotypes adapted to either low- or high-light niches2,3. The pcb genes, encoding the major chlorophyll-binding, light-harvesting antenna proteins in this genus4, are present in multiple copies in low-light strains but as a single copy in high-light strains5. The basis of this differentiation, however, has remained obscure. 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 photosystem I (PSI)-comparable to isiA genes from cyanobacteria6,7-and a constitutively expressed pcb gene encoding a photosystem II (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 algae8.
In order to gain a better understanding of the origin, function and localization of the divinyl-chlorophyll a/b-binding antenna complexes of Prochlorococcus species and how these properties relate to the light niche to which different strains are adapted, we have undertaken gene expression and structural studies on the moderate low-light-adapted strain MIT 9313 for which the full genome sequence is available (http://www.jgi.doe.gov/JGI_microbial/html/). This strain contains two pcb genes: pcbA (PMT 1046) and pcbB (PMT 0496). This contrasts with the very low-light-adapted strain SS120 that contains eight pcb genes (pcbA to pcbH)-analysis of its genome sequence (http://www.sb-roscoff.fr/Phyto/ProSS120/) revealed the presence of one more pcb gene (pcbH) than previously thought5-and the high-light-adapted strain MED4, which has only a single pcb gene (pcbA)4,5.
Recently it was shown by electron microscopy and single-particle analyses that SS120 contains a giant supercomplex consisting of the PSI reaction centre trimer surrounded by a light-harvesting antenna ring composed of 18 Pcb subunits9, similar to the 18-mer IsiA-PSI supercomplex induced in cyanobacteria when deprived of iron6,7. However studies on MIT 9313 grown under similar conditions to SS120 did not reveal the presence of an 18-mer Pcb-PSI supercomplex but only 'naked' trimeric PSI complexes, which matched with the cyanobacterial...