Genes and transcripts for the polypeptides of the cytochrome b6/f complex from spinach thylakoid membranes

Abstract

Cytochrome b6/f complex was prepared from washed thylakoid membranes by a procedure involving detergent treatment and centrifugation in sucrose gradients. The complex is composed of at least four polypeptide species, cytochrome f which occurs in two variant forms (mol. wt. 34/33 kd), cytochrome b6 (23 kd), the high‐potential Rieske iron‐sulfur protein (19 kd) and a fourth subunit (17 kd) of unknown function. Transcripts for the cytochromes f, b6 and subunit 4 were found in plastid RNA, those for the Rieske iron‐sulfur protein in cytosolic poly(A)⁺ RNA. Transcripts for cytochrome b6 and subunit 4 are translated in rabbit reticulocyte lysates into products of correct length. The Rieske iron‐sulfur protein and the cytochrome f apoprotein appear to be made as precursors with excess sequences of 7 and 4 kd, respectively. Cytochrome f, cytochrome b6 and subunit 4 are encoded by uninterrupted plastid genes that are located in the large single‐copy region of the circular DNA molecule. Each of these genes is present once per chromosome. Their location and direction of transcription have been determined by hybrid‐selection mapping and by cell‐free transcription/translation of various recombinant DNAs. The genes for cytochrome b6 and for subunit 4 lie near each other, but do not overlap. They are transcribed into a single message. The gene for cytochrome f maps 15 kbp away from this cluster, close to the 3′ end of the gene for the large subunit of ribulosebisphosphate carboxylase/oxygenase, and is transcribed into a separate 4 kb long RNA. All these genes have the same polarities with respect to each other.