Phycobiliproteins in Prochlorococcus marinus: biosynthesis of pigments and their assembly into proteins

Prochlorococcus sp. is a very unique and highly abundant class of organisms within the cyanobacteria. Found in the world's oceans Prochlorococcus is very small in size and possesses the smallest genome of a photosynthetic autotroph. Prochlorococcus is characterized by a special chlorophyll antenna for light harvesting and the absence of classical cyanobacterial phycobilisomes. Despite the lack of phycobilisomes Prochlorococcus possesses remnants thereof which is the phycobiliprotein phycoerythrin (PE) encoded in a PE operon as well as genes encoding enzymes of phycobilin biosynthesis. The size of this PE operon varies depending on the light-adapted ecotype. While high-light strains only possess a β-subunit of PE, low-light adapted strains possess both, an α- and a β-subunit. α-/β-subunits are also present in functional phycobilisomes. Consistent with the number of subunits is also the varying number of putative lyase genes, involved in the transfer and attachment of phycobilins (open-chain tetrapyrroles) to the PE subunits. This minireview summarizes the only sparely available data on the biosynthesis and assembly of Prochlorococcus PE. On one hand the quite well understood biosynthesis of pigments will be reviewed but also new data on the phycobiliprotein lyase-mediated transfer of the phycobilins to the PE subunits will be discussed.