The subunit composition of highly active purified ATP-synthase from chloroplasts, CF0F1, was investigated by SDS gel electrophoresis. An additional subunit of CF0, was detected with an apparent molecular mass of 20 kDa. It is stained weakly with Coomassie blue but very strongly with silver. This subunit was isolated on a preparative SDS gel and the N-terminal amino acid sequence analyzed. It shows that the 20 kDa protein is identical with the protein encoded by the spinach chloroplast gene atpI, called subunit IV [(1986) Mol. Genet. 203, 117–128]. However, in comparison to the gene-derived sequence, the first 18 amino acids are missing, indicating N-terminal processing.
