Diuron Resistance in the PSBA Multigene Family of Anacystis Nidulans R2

Abstract

Atrazine-resistant strains of many higher plants (1, 2, 3) and algae (4, 5) have been isolated. In Amaranthus hvbridus (2), Solanum nigrum (3), and Chlamvdomonas reinhardtii (4) resistance has been correlated with a single basepair change at nt 790 of the psbA gene which corresponds to a ser→ gly or ser→ ala change at aa 264 of the psbA gene product— the QB protein. The QB protein, also known as the 32 kilodalton or D1 jprotein, is thought to provide a binding pocket for plastoquinones involved in QA→ QB electron transfer of photosystem II. Anacvstis nidulans R2 (PCC# 7942) is a transformable cyanobacterium which performs oxidative photosynthesis similar to higher plants. Diuron-resistant, (D^r), strains of A. nidulans. which are also resistant tc atrazine, have been isolated after nitrosoguanidine mutagenesis (6). These strains have the analogous mutation in the psbAI gene resulting in a ser→ ala change at aa 264 of the QB protein. Transformation of wild type A. nidulans with an internal uNA fragment of this resistant psbA gene yields D^r strains, demonstrating that the change at aa 264 of the QB protein is sufficient to confer diuron-resistance. In A. nidulans there are three functional copies of the psbA gene (Golden, S.S., Brusslan, J.A., and R. Haselkom, submitted). The original D^r strain isolated by S. S. Golden (6) contains a mutant psbAI gene, while psbAII and psbAIH remain wild type, diuron-sensitive (D^s). The psbAI transcript accounts for 95% of the total psbA mRNA (Golden et al, submitted) so it might be expected that mutation of this copy, alone, could confer a D^r phenotype because most of the reaction centers would contain D^r QB proteins.