Thanks James! I quickly prepared an overlay with 3zpn (gray and blue). The CP47 C-terminus binds to a part of the Psb28 surface, which is also involved in dimer formation. It seems to make sense that we see only the monomer.

We speculate that this site most likely corresponds to the long-sought single high-affinity site (HAS), where the first Mn2+ binds prior to the first photoactivation step in OEC biogenesis. 22/23

Surprisingly, we see another density in the area where the cluster is located in mature PSII, most likely of a single, positively charged ion, which is coordinated by the side chains of D1 Asp170, Glu189, and His332. 21/23

A closer look at the potential cluster binding site in PSII-I reveals interesting differences compared to mature PSII. The position of the D1 C-terminus, an important structural element of the cluster binding site, is clearly immature. 20/23

However, the binding position supports the hypothesis that Psb27 stabilizes the CP43 module prior to attachment, thus facilitating its association with RC47, which is very important for activation of the water-oxidizing cluster. 19/23

Psb27 binds in a remote position to loop E of CP43 at the luminal PSII surface. This position has only little overlap with the binding sites of the extrinsic proteins (PsbO, PsbU, PsbV) and no direct contact to the binding site of the Mn4CaO5 cluster. 18/23

Consequently, charge recombination is altered in PSII-I leading to reduced production of singlet oxygen, a very harmful reactive oxygen species, which indicates a novel protection mechanism that is operating during PSII biogenesis. 17/23

Most surprising, Psb28 binding reorganizes the structure of the D1 D-E-loop in a way that it resembles the coordination of the non-heme iron in reaction centers of non-oxygenic photosynthetic bacteria. D2 Glu241 replaces bicarbonate as ligand of the iron between QA and QB. 16/23

The QB binding site is distorted and most likely unable to bind quinone, as D1 Phe265, which binds to the quinone head group, is clearly at a different position. 15/23

The binding of CP43 is immature. It ‘clicks’ into place after release of Psb28 and Psb34. 14/23