ATAB2 is a novel factor in the signalling pathway of light-controlled synthesis of photosystem proteins.

Plastid translational control depends to a large extent on the light conditions, and is presumably mediated by nucleus-encoded proteins acting on organelle gene expression. However, the molecular mechanisms of light signalling involved in translation are still poorly understood. We investigated the role of the Arabidopsis ortholog of Tab2, a nuclear gene specifically required for translation of the PsaB photosystem I subunit in the unicellular alga Chlamydomonas. Inactivation of ATAB2 strongly affects Arabidopsis development and thylakoid membrane biogenesis and leads to an albino phenotype. Moreover the rate of synthesis of the photosystem reaction center subunits is decreased and the association of their mRNAs with polysomes is affected. ATAB2 is a chloroplast A/U-rich RNA-binding protein that presumably functions as an activator of translation with at least two targets, one for each photosystem. During early seedling development, ATAB2 blue-light induction is lowered in photoreceptor mutants, notably in those lacking cryptochromes. Considering its role in protein synthesis and its photoreceptor-mediated expression, ATAB2 represents a novel factor in the signalling pathway of light-controlled translation of photosystem proteins during early plant development.