A subset of cyanobacteria has evolved to use far-red light (FRL) to support their photosynthetic growth in a process known as far-red light photoacclimation (FaRLiP)1. FaRLiP involves the synthesis and incorporation of FRL-absorbing chlorophyll (Chl) molecules (Chl f and Chl d) into a new set of FRL-absorbing photosystems to allow oxygenic photosynthesis to be driven by lower energy photons2.
Bryant and colleagues showed that a divergent D1 paralog found in FaRLiP species, referred to as super-rogue D13 (srD1) or ChlF4, is required for light-induced Chl f biosynthesis4.
We subsequently showed that srD1 expressed heterologously in Synechocystis PCC 6803 assembles into a variant PSII complex, termed the super-rogue photosystem II complex (srPSII), that drives Chl f synthesis but cannot oxidize water5.
Here I describe the cryo-EM structure of a His-tagged srPSII complex obtained at a resolution of 2.46Å. The monomeric complex contains the srD1 subunit encoded by Chroococcidiopsis thermalis PCC 7203 and 15 of the 17 intrinsic subunits found in oxygen-evolving PSII. Missing are the extrinsic PsbO, PsbU, PsbV and CyanoQ subunits on the luminal side of the complex and the intrinsic PsbJ and PsbY subunits. Present in the srPSII complex are 35 Chls (with 3 predicted to be Chl f by HPLC), 10 carotenoids, plastoquinones QA and QB, the non-heme iron but there is no evidence for the binding of Ca2+ or Mn ions. The cryo-EM structure of srPSII reveals altered binding of CP43 to srD1 which might be related to the role of srPSII in Chl f biosynthesis. The mechanism is unknown but might involve reactive oxygen species produced by srPSII in the light.