Study of Reaction Centers From RB. Capsulatus Mutants Modified in the QB Binding Site

Abstract

In photosynthetic reaction centers, herbicides block electron transfer between primary and secondary quinone acceptors, QA and QB, respectively, by competing with QB for its proteic binding site. The similitude between the reaction center from purple bacteria, which 3D structure is known at 3 A resolution, and that of PSII of plants, still unknown, validates comparative studies concerning herbicides binding. Herbicide-resistant mutants from different bacterial species have been either designed by site directed mutagenesis or selected on their ability to grow in the presence of herbicides. This was done for Rb. capsulatus (Bylina and Youvan, 1987; Bylina et al, 1989), Rb.sphaeroides (Paddock et al., 1988) and Rps. viridis (Sinning and Michel, 1987). Among all these mutants, it has been pointed out that the replacement of IleL229 in Rb. sphaeroides and Rb. capsulatus is of special interest. From the 3D structure of the reaction center from Rps. viridis with s-triazine herbicide terbutryn bound in the QB pocket, it appears that IleL229 makes extensive contacts with that inhibitor and QB. Depending on the nature of the substitution in Rb. capsulatus, the mutations lead to various levels of photosynthetic growth and of terbutryn resistance compared to the wild type (Bylina and Youvan, 1987). Since assays have been done on whole cells it is likely that combination of different factors is at work in modulating the photosynthetic activities of the mutants.