Identification of the triazine receptor protein as a chloroplast gene product

The triazine herbicides inhibit photosynthesis by blocking electron transport at the second 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 has recently been identified as a 32- to 34-kilodalton polypeptide by using a photoaffinity labeling probe, azido-[¹⁴ C]atrazine. A 34-kilodalton polypeptide of pea 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. From the identical pattern of susceptibility to trypsin we conclude 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.