Identification of the triazine receptor protein as a chloroplast gene product

Abstract

The triazine herbicides inhibit photosynthesis by blocking electron transport at the 2nd stable electron acceptor of photosystem II. This electron transport component of chloroplast thylakoid membranes is a protein-plastoquinone complex termed "B." The polypeptide that is believed to be a component of the B complex was identified as a 32-34-kilodalton polypeptide by using a photoaffinity labeling probe, azido[14C]atrazine. A 34-kilodalton polypeptide of pea [Pisum sativum] chloroplasts rapidly incorporates [35S]methionine in vivo and is also a rapidly labeled product of chloroplast-directed protein synthesis. Trypsin treatment of membranes tagged with azido-[14C]atrazine, [35S]methionine in vivo, or [35S]methionine in isolated intact chloroplasts results in identical, sequential alterations of the 34-kilodalton polypeptide to species of 32, then 18 and 16 kilodaltons. The identical pattern of susceptibility to trypsin indicates that the rapidly synthesized 34-kilodalton polypeptide that is a product of chloroplast-directed protein synthesis is identical to the triazine herbicide-binding protein of photosystem II. Chloroplasts of both triazine-susceptible and triazine-resistant biotypes of Amaranthus hybridus synthesize the 34-kilodalton polypeptide, but that of the resistant biotype does not bind the herbicide.