Synthesis of ATP synthase

Chloroplast ATP synthase is present in dark-grown plants, indicating that light is not essential for the synthesis of any of the subunits of this complex. However, the regulatory features necessary to coordinate the expression of chloroplast and nuclear genes are not yet fully understood. One feature that needs to be investigated in more detail is the provision of sufficient subunits for assembly in the stoichiometry demanded by the ATP synthase complex. The necessary subunit stoichiometry cannot be provided solely by transcriptional control mechanisms, and regulation at translational or post-translational steps will also be required. 

The chloroplast genes for the CFq subunits IV, III and I and for the a subunit of CFj are co-transcribed to give a primary transcript which is then processed to remove the intron in atp and to generate a number of smaller transcripts [148]. Nothing is known of the mechanisms of these processing events. The chloroplast genes for the /3 and e subunits of CFx are also co-transcribed [151,152]. The transcripts are translated on chloroplast ribosomes, as indicated by the sensitivity of 

The 7 and d subunits of CFi and subunit II of CFq are synthesized on cytoplasmic ribosomes, as indicated by the sensitivity of their synthesis to cyclohex-imide in pea [163] and Spirodela [121]. Each of these subunits has been shown to be synthesized as a higher-Mj form on translation of poly(A) RNA in vitro [142,146]. The y subunit is synthesized initially as a 46 kDa form, the 8 subunit as a 27 kDa form and subunit II as a 24 kDa form. These precursors are 7-9 kDa larger than the mature polypeptides, and probably contain N-terminal transit peptide sequences necessary for the import of the polypeptides into the chloroplasts. However, this has not been established for any of the nuclear-encoded subunits of ATP synthase. 

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