A novel protein domain is important for photosystem Ⅱ complex assembly and photoautotrophic growth in angiosperms

Photosystem Ⅱ(PSⅡ)is a multi-subunit protein complex of the photosynthetic electron transport chain that is vital to photosynthesis.Although the structure,composition,and function of PSⅡ have been extensively studied,its biogenesis mechanism remains less understood.Thylakoid rhodanese-like(TROL)provides an anchor for leaf-type ferredoxin:NADP^(+)oxidoreductase.Here,we report the chacterizaton of a second type of TROL protein,TROL2,encoded by seed plant genomes whose function has not previously been reported.We show that TROL2 is a PSⅡ assembly cofactor with essential roles in the establishment of photoautotrophy.TROL2 contains a 45-amino-acid domain,termed the chlorotic lethal seedling(CLS)domain,that is both necessary and sufficient for TROL2 function in PSⅡ assembly and photoautotrophic growth.Phylogenetic analyses suggest that TROL2 may have arisen from ancestral TROL1 via gene duplication before the emergence of seed plants and acquired the CLS domain via evolution of the sequence encoding its N-terminal portion.We further reveal that TROL2(or CLS)forms an assembly cofactor complex with the intrinsic thylakoid membrane protein LOW PSⅡ ACCUMULATION2 and interacts with small PSⅡ subunits to facilitate PSⅡ complex assembly.Collectively,our study not only shows that TROL2(CLS)is essential for photoautotrophy in angiosperms but also reveals its mechanistic role in PSⅡ complex assembly,shedding light on the molecular and evolutionary mechanisms of photosynthetic complex assemblyin angiosperms.