Characterization of the PetM subunit of the b(6)f complex from higher plants

The chloroplast cytochrome b(6)f complex comprises seven subunits, three of which - designated the PetG, PetL and PetM proteins - are very small. To d etermine structural data on the PetM protein from higher plants the spinach b(6)f complex was isolated by a novel isolation procedure based on blue na tive polyacrylamide gel electrophoresis (BN-PAGE). Starting with only 30 mg of chloroplast protein the enzyme complex is obtained in very pure form as demonstrated by analysis with Tricine-SDS/PAGE. The primary structure of t he PetM subunit was investigated by direct amino acid sequencing and by seq uencing of a corresponding cDNA clone from Arabidopsis thaliana. Arabidopsi s PetM comprises 40 amino acids and is synthesized with a N-terminal extens ion of 56 amino acids that resembles stroma-targeting peptides. Hence, the PetM protein is assumed to be nuclear encoded and posttranslationally impor ted into chloroplasts. Arabidopsis PetM exhibits 58 % sequence identity wit h PetM from Chlamydomonas. Furthermore, Arabidopsis PetM resembles the dedu ced amino acid sequences of small organelle-encoded open reading frames of the red alga Porphyra purpurea, the diatom Odontella sinensis, the green al ga Cyanophora paradoxa and the cyanobacterium Synechocystis. The primary st ructure of PetM does not: show any significant sequence similarity to the s equences of one of the small subunits of the mitochondrial bc(1) complex. T he overall subunit compositions of chloroplast b(6)f complexes and mitochon drial bc(1) complexes are compared and discussed.