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 PTAC12 and PTAC7. Mostly expressed in leaves, flowers and seedlings, and, to a lower extent, in stems and roots. By light. Seedling lethal, even with Suc as a carbon source. Impaired thylakoid formation in the initial process of chloroplast development leading to albino seedlings unable to grow photoautotrophically. Reduced plastid-encoded RNA polymerase (PEP) activity. Belongs to the class V-like SAM-binding methyltransferase superfamily.