The 4-kDa nuclear-encoded PetM polypeptide of the chloroplast cytochrome b(6)f - Complex nucleic acid and protein sequences, targeting signals, transmembrane topology

The 4-kDa subunit of cytochrome b(6)f complex encoded by the nuclear PetM gene in Chlamydomonas reinhardtii has been characterized, 38 of the 39 residues of the mature protein have been established by Edman degradation, a cDNA clone encoding the complete precursor has been isolated and sequenced, and a 0.6-kb transcript detected, The deduced amino acid sequence of the precursor includes an N-terminal transit peptide of 60 amino acids with stromal targeting features. Examination of the sequence suggests that PetM spans the membrane as a single transmembrane alpha-helix, which is supported by its non-extractability following dissociating treatments, When PetM and PetG, another small subunit of the b(6)f complex, are folded into alpha-helices, an array of identical residues becomes apparent, Proteolysis data, charge distribution, and homology with PetG are consistent with a lumenal localization of the N terminus of PetM.