Oxygenic photosynthesis relies on accessory factors to promote the assembly and maintenance of
the photosynthetic apparatus in the thylakoid membranes. The highly conserved membrane-bound
rubredoxin-like protein RubA has previously been implicated in the accumulation of both
photosystem I (PSI) and photosystem II (PSII) but its mode of action remains unclear. Here we
show that RubA in the cyanobacterium Synechocystis sp. PCC 6803 is required for
photoautotrophic growth in fluctuating light and acts early in PSII biogenesis by promoting the
formation of the heterodimeric D1/D2 reaction center complex, the site of primary photochemistry.
We find that RubA, like the accessory factor Ycf48, is a component of the initial D1 assembly
module as well as larger PSII assembly intermediates and that the redox-responsive rubredoxinlike domain is located on the cytoplasmic surface of PSII complexes. Fusion of RubA to Ycf48
still permits normal PSII assembly suggesting a spatiotemporal proximity of both proteins during
their action. RubA is also important for the accumulation of PSI but this is an indirect effect
stemming from the downregulation of light-dependent chlorophyll biosynthesis induced by PSII
deficiency. Overall our data support the involvement of RubA in the redox control of PSII
biogenesis.