Essential for chloroplast development, especially for thylakoid formation. Involved in plastid gene expression, probably by maintaining plastid-encoded RNA polymerase (PEP) activity. Component of the transcriptionally active chromosome (TAC) complexes. Interacts with FLN1, PTAC10, PTAC12 and PTAC14. Mostly expressed in leaves, flowers and seedlings, and, to a lower extent, in roots and stems. By light. Albino plants with defective chloroplasts containing a few appressed membranes but lacking thylakoids, even when grown under normal light conditions. Reduced plastid-encoded RNA polymerase (PEP) activity.