The Herbicide Resistant Mutant T1 from Rhodopseudomonas viridis Altered Herbicide Binding and Three-Dimensional Structure

Herbicides of the triazine class are known to act by competing with the secondary acceptor, QB, for binding to the photosynthetic reaction center (RC) of purple bacteria and PSII (1,2). The binding of terbutryn to the RC of the purple bacterium Rhodopseudomonas (Rps.) viridis wild type (WT) was established by X-ray crystallography (3). Several hydrogen bonds and numerous van der Waals interactions contribute to the binding of the inhibitor. There is a hydrogen bond possible between the ethylamino group of terbutryn and the hydroxo group of serine L223. A second hydrogen bond is likely between a nitrogen atom of the triazine ring and the peptide nitrogen of isoleucine L224. Several mutants, which are resistant toward terbutryn, have been isolated by selecting for photosynthetic growth in the presence of the herbicide (4,5). In the mutant Tl (SerL223→Ala and Arg L217→His) binding of the secondary acceptor, QB, is increased compared to the wild type (5). Due to significant sequence homologies, structural and mechanistic similarities, the RC of purple bacteria is an interesting model for PSI