Chloroplast Cytochrome B6f Complex Research Articles

The complete amino-acid sequence of the rieske FeS-precursor protein from spinach chloroplasts deduced from cDNA analysis

Summary Several cDNA clones encoding the entire Rieske FeS-precursor protein of the chloroplast cytochrome b 6 f-complex have been isolated by high density plaque immunoscreening of a phage lambda gt11 cDNA expression library, made from poly A+-RNA of spinach seedlings. The identity of the cDNAs has been confirmed by N-terminal amino acid sequencing of the purified protein. The nucleotide sequence indicates a protein of 247 amino acid residues including a putative transit sequence of 68 amino acids corresponding to molecular masses of 26.3 kDa (precursor) and 18.8 kDa (mature protein; 179 amino acid residues). Alignteins of the sequence with sequences from Rieske FeS-proteins of respiratory electron transport chains, two of bacterial and three of mitochondrial origin, shows little sequence homology, but remarkable similarity in secondary structure including a putative N-terminal transmembrane segment of about 25 residues and the peptides CTHLGCV and CPCHGS in the C-terminal region of the protein that are involved in the binding of the Fe2S2-cluster.

Functional size measurements on the chloroplast cytochrome bf complex

Radiation inactivation studies on the functional size of the chloroplast cytochrome bf complex showed the following characteristics. (1) In the purified complex, the cytochrome b-563, cytochrome f and Rieske polypeptides gave independent target sizes close to those obtained from gene sequence data. (2) The molecular mass required for electron transport from plastoquinol-1 to plastocyanin in chloroplast membranes was approximately 75 kDa. This indicates that one copy of each major polypeptide forms a functional complex. (3) Measurements on the Rieske EPR signal indicate that the binding of the inhibitor 2,5-dibromo-6-methyl-3-isopropyl- p-benzoquinone (DBMIB) involves more than one polypeptide. It is argued that the quinone-binding site blocked by DBMIB involves both the Rieske and cytochrome b polypeptides.

An EPR study of the lateral organization of electron carriers in chloroplast thylakoids

Recent evidence on photosynthetic electron transport indicates the organization of electron carriers into 3 supramolecular intrinsic complexes: photosystem I (PSI); photosystem II (PSII); and the cytochrome b6-f complex [l]. The complexes are linked by mobile electron carriers, plastoquinone and plastocyanin. Most higher plant chloroplasts also contain two different types of thylakoid membranes: appressed membranes of the grana partitions: and stroma-exposed, non-appressed thylakoids. It has been proposed that there is a lateral heterogeneity of two of the electrontransfer complexes among the different regions of the thylakoid membrane with PSI1 complex primarily located in appressed thylakoids and PSI complex mainly located in non-appressed stroma thylakoids [2-51. The chloroplast cytochrome b6-f complex has been reported to show a more even distribution along these two membrane regions ]6,71. known as signal II [lo]. The cytochrome b6--f complex has been analyzed using the EPR signal of the reduced Rieske iron-sulfur center [ 111. Our results support the view that the PSI and PSI1 complexes are mainly spatially separated in different membrane regions, while the cytochrome complex is present in both appressed and non-appressed membranes.