Identification of the initiation codon for the atpB gene in Chlamydomonas chloroplasts excludes translation of a precursor form of the β subunit of the ATP synthase

The chloroplast atpB gene of Chlamydomonas reinhardtii, which encodes the β subunit of the ATP synthase, contains three in-frame ATGs that are candidate translation initiation codons. An earlier study revealed that the N terminus of the assembled β subunit maps at the +2 position with respect to the second in-frame methionine codon (Fiedler et al. 1995). Using chloroplast transformation, we have examined the possibility that either of the two additional in-frame ATG codons is competent for translation initiation. We provide evidence that translation of atpB is initiated exclusively at the second ATG codon. We conclude that the β subunit is not synthesized with an N-terminal leader before its assembly into a functional ATP synthase complex.