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

The chloroplast atpB gene of Chlamydomonas reinhardtii, which encodes the b eta subunit of the ATP synthase, contains three in-frame ATGs that are cand idate translation initiation codons. An earlier study revealed that the N t erminus of the assembled beta subunit maps at the +2 position with respect to the second in-frame methionine codon (Fiedler et al. 1995). Using chloro plast transformation we have examined the possibility that either of the tw o additional in-frame ATG codons is competent for translation initiation. W e provide evidence that translation of atpB is initiated exclusively at the second ATG codon. We conclude that the beta subunit is not synthesized wit h an N-terminal leader before its assembly into a functional ATP synthase c omplex.