Phycocyanobilin biosynthesis in Galdieria sulphuraria requires isomerization of phycoerythrobilin synthesized by bilin reductases

Abstract

Cyanobacteria and red algae (Rhodophyta) utilize phycobilisomes as light-harvesting antennae to carry out oxygenic photosynthesis. Phycobilisomes are large protein complexes comprised of phycobiliproteins and associated linker proteins, with phycobiliproteins carrying one or more linear tetrapyrrole (bilin) chromophores. Bilin biosynthesis is typically catalyzed by ferredoxin-dependent bilin reductases (FDBRs). Cyanobacterial genomes encode one FDBR, PcyA, for biosynthesis of phycocyanobilin (PCB), which is required both for harvesting red light and for coupling phycobilisomes to photosynthetic reaction centers via energy transfer. Some cyanobacteria also contain two additional FDBRs (PebA and PebB) for synthesis of the green-absorbing phycoerythrobilin (PEB). Most rhodophytes contain only PebA and PebB homologs (PEBA and PEBB), so the biosynthesis of PCB in such organisms is not understood. This process is especially relevant in the early-branching rhodophyte Galdieria sulphuraria, which contains PCB as the main light-harvesting chromophore yet contains the PEBA and PEBB FDBRs for PEB biosynthesis. In this organism, PCB biosynthesis is thought to be dependent both on FDBRs and on a yet-to-be identified bilin isomerase. Our current work reaffirms these findings. Phylogenetic analysis clearly places G. sulphuraria FDBRs in the PEBA and PEBB clades, in contrast to the PCYA found in Cyanidioschyzon merolae, another early-branching rhodophyte. All three rhodophyte FDBRs were found to have the expected substrate preferences and enzymatic activities after recombinant expression, affinity purification, and in vitro characterization. Therefore, bilin biosynthesis in G. sulphuraria results in biosynthesis of PEB rather than PCB. However, a previous procedure allowed partial purification of a PEB:PCB isomerase, clearly demonstrating the existence of such an activity and adding to our understanding of bilin biosynthesis and algal light harvesting.