Photosystem I reaction center polypeptides of spinach are synthesized on thylakoid-bound ribosomes.

Purified chloroplasts were prepared from developing spinach leaves. The chloroplasts were separated into thylakoid and stroma fractions, and nucleic acids were prepared from them. Photosystem I reaction center polypeptide(s) (PS I RC) mRNA was associated with the thylakoid fraction when measured by hybridization using a probe for PS I RC polypeptide ps1A1, or when measured by translation assay. The ps1A1 polypeptide was coded for by a 5.5-kbp mRNA which others have shown also codes for PS IRC polypeptide ps1A2. This mRNA was in functional thylakoid-bound ribosomes because when thylakoids with bound ribosomes were translated in the absence of protein synthesis initiation, polypeptides that reacted with anti-PS I RC were formed. The results indicate that PS I RC polypeptides are synthesized exclusively by thylakoid-bound ribosomes.