Phycobilin biosynthesis for light harvesting in the cryptophyte alga Hemiselmis cryptochromatica

In cyanobacteria, chromatic acclimation (CA) is a well-studied process in which the organism adapts its light-harvesting machinery to varying light qualities. Therefore, the phycobilisomes (PBS) get restructured, depending on the type of CA used. A similar process was recently described for the cryptophyte alga Hemiselmis cryptochromatica. Cryptophytes are microalgae, originating from a secondary endosymbiosis event, where a red alga cell was ingested by an unknown eukaryote and are capable of living in a variety of aquatic habitats. For light harvesting, phycobiliproteins (PBPs) are used, which in contrast to cyanobacteria are present as soluble proteins in the thylakoid lumen. Color differences amongst cryptophytes depend on the type of PBP pigment. In total, nine major differently absorbing PBPs have been identified so far. The PBPs can either be phycocyanin (PC) or phycoerythrin (PE), designated by the maximum absorption wavelength. Cryptophyte PBPs are soluble α-/β-hetero dimers, with the β-subunit highly homologous to the ancestral red algal one. The α-subunit, in contrast, evolved separately. Overall, multiple α-subunits are encoded on the nucleus genome, while one β-subunit is encoded on the plastid genome. α-subunits covalently bind one light absorbing linear tetrapyrrole (phycobilin) and β-subunits have three phycobilins covalently attached. In contrast to other organisms, cryptophytes possess rather unusual phycobilins incorporated in their PBPs. These are 18¹,18²-dihydrobiliverdin (MBV) and 15,16-dihydrobiliverdin (DHBV) and the unique phycobilins bilin 584 and bilin 618. They are acryoyl-derivatives of PEB and DHBV and their biosynthesis is still unknown. The project aims at providing deeper understanding for the molecular mechanisms of the observed CA. In particular, we will isolate and identify the PBP chromophores under different light qualities, as well as look at their biosynthetic pathway. We bioinformatically identified five FDBRs in the H. cryptochromatica transcriptome which are now investigated in a recombinant protein approach. Results indicate the production of PCB by two of the encoded FDBRs showing at least one possible function while the functions of the other three remains to be elucidated. Overall, this study will help in understanding the specialized phycobilin biosynthesis and CA in cryptophytes.