InterPro domain: IPR036062

Oxygenic photosynthesis uses two multi-subunit photosystems (I and II) located in the cell membranes of cyanobacteria and in the thylakoid membranes of chloroplasts in plants and algae. Photosystem II (PSII) has a P680 reaction centre containing chlorophyll 'a' that uses light energy to carry out the oxidation (splitting) of water molecules, and to produce ATP via a proton pump. Photosystem I (PSI) has a P700 reaction centre containing chlorophyll that takes the electron and associated hydrogen donated from PSII to reduce NADP+ to NADPH. Both ATP and NADPH are subsequently used in the light-independent reactions to convert carbon dioxide to glucose using the hydrogen atom extracted from water by PSII, releasing oxygen as a by-product.

This superfamily consists of the photosystem I reaction centre subunit IX or PsaJ from various organisms including Synechocystis sp. (strain PCC 6803), Pinus thunbergii (Green pine) and Zea mays (Maize).PsaJ ( P19443 ) is a small 4.4kDa, chloroplast encoded, hydrophobic subunit of the photosystem I reaction complex whose function is not yet fully understood [ 1 ]. PsaJ can be cross-linked to PsaF ( P12356 ) and has a single predicted transmembrane domain. It has a proposed role in maintaing PsaF in the correct orientation to allow for fast electron transfer from soluble donor proteins to P700+ [ 2 ].

1. A large fraction of PsaF is nonfunctional in photosystem I complexes lacking the PsaJ subunit. Biochemistry 38, 5546-52