Nuclear Genes Encoding Plastid Proteins
The majority of plastid localized proteins are encoded by nuclear genes. These genes are transcribed by RNA polymerase II and the resultant transcripts are spliced, capped and polyadenylated in the nucleus. The mRNAs then are translated by 80S ribosomes in the cytoplasm to produce proteins which can be transported into plastids posttranslationally.

Following uptake into the chloroplast, the proteins are assembled with cofactors and other proteins to form functional complexes, Control sequences at the 5' -end of the gene which are involved in initiating transcription are very similar to the "Pribnow': box and the '-35' region characteristic of bacterial genes.

After synthesis, the large subunit (which has limited solubility) is probably bound by a stabilizing protein to maintain solubility prior to assembly into active enzyme. The small subunit is synthesized in the cytoplasm, on free ribosomes, with an N-terminal leader peptide of about 20 amino acids.

The complete peptide is then taken up into the chloroplast in an ATP dependent manner, accompanied by removal of the leader peptide by a stromal peptidase. This mechanism is not analogous to that involving signal peptide cleavage. The synthesis of this important enzyme clearly depends on the coordinate expression of genes in different genomes.