Phycoerythrin and the photosynthetic apparatus of Prochlorococcus: Insights through comparative genomics

Abstract

Prochlorococcus is a chlorophyll b-containing cyanobacterium. Ecotypes which dominate the surface or deeper waters have different irradiance optima of 150 or 3 m moles photons m-2 s-1, respectively. Complete genome sequences have been determined of two Prochlorococcus strains, MED4 and MIT9313, representative of high and low light-adapted ecotypes, respectively. Both genomes are highly compact and are significantly smaller than those of more classical cyanobacteria, but their gene content has substantially diverged. Thus, the comparative analysis of these genomes provides information on the genetic basis of the differences in photosynthetic performance. In addition it demonstrates the evolutionary adaptations taking place on a relatively small time scale: MIT9313 is genetically close to classical cyanobacteria whereas MED4 is farther evolved. MED4 has more genes encoding putative high light inducible proteins, or photolyases to repair UV induced DNA damage. In contrast, MIT9313 possesses more genes to build up light harvesting structures. These are two pcb genes encoding DV-chlorophyll binding proteins as well as a gene cluster to produce chromophorylated phycoerythrin. The latter represents an evolutionary intermediate between classical phycobiliproteins and an extremely diverged b phycoerythrin present in MED4. Intriguing similarities between both Prochlorococcus strains include a gene cluster for Rubisco and carboxysomal proteins that is of obviously non-cyanobacterial origin, and genes for the formation of carotenoids that are common in green organisms but not in cyanobacteria.