The family of chloroplast ALBINO3 proteins function in the insertion and assembly of thylakoid 40 membrane protein complexes. Loss of ALB3b in the marine diatom Phaeodactylum tricornutum

39 The family of chloroplast ALBINO3 proteins function in the insertion and assembly of thylakoid 40 membrane protein complexes. Loss of ALB3b in the marine diatom Phaeodactylum tricornutum 41 leads to a striking change of cell color from the normal brown to green. A 75% decrease of the 42 main fucoxanthin-chlorophyll a/c-binding proteins was identified in the alb3b strains as the 43 cause of changes in the spectral properties of the mutant cells. The alb3b lines exhibit a truncated 44 light-harvesting antenna phenotype with reduced amounts of light harvesting pigments and 45 require a higher light intensity for saturation of photosynthesis. Accumulation of photoprotective 46 pigments and LHCX proteins were not negatively affected in the mutant strains, but still the 47 capacity for non-photochemical quenching was lower compared to wild type. In plants and green 48 algae, ALB3 proteins interact with members of the chloroplast signal recognition particle 49 pathway through a lysine-rich C-terminal domain. A novel conserved C-terminal domain was 50 identified in diatoms and other stramenopiles, questioning if ALB3b proteins have the same 51 interaction partners as its plant/green algae homologs. 52