Essential for photosystem II (PSII) biogenesis; required for assembly of an early intermediate in PSII assembly that includes D2 (psbD) and cytochrome b559. Has been suggested (PubMed:11826309) to be required for chlorophyll a binding. Interacts with PAM68. Restricted to the stromal lamellae (PubMed:9736608). Translocation into the thylakoid lumen occurs via the Tat pathway (Probable) (PubMed:10664464). Expression in green tissue, not roots. Accumulates also in dark-grown seedlings. A 7-bladed beta-propeller torus, about 54 by 55 Angstroms, with a depth of about 25 Angstroms and a central pore. Seedling lethal when homozygous, has pale cotyledons but never develops true leaves. On sucrose-supplemented medium PSII is completely inactive; D1, D2 CP43 and CP47 are present in very low amounts. Belongs to the Ycf48 family.