Partial molecular analysis of the psbA gene in Euglena gracilis mutants exhibiting resistance to DCMU and atrazine.

Mutations conferring herbicide resistance have been detected in two new strains (ZR250 and ZR480) of Euglena gracilis Z by partial gene cloning and sequencing. These mutants were originally derived from Z cells grown in medium containing progressively increasing concentrations of DCMU. Each of these strains have been characterized by measuring their growth kinetics, O2 evolution, and resistance to DCMU and atrazine. Partial sequences of the psbA gene of these strains were compared to those published for strains Z and ZR25. The ZR250 and ZR480 strains were found to be double mutants. Besides the expected mutation S265A, they showed an additional point mutation at codon 219 (equivalent to codon 218 of other organisms). This mutation results in leucine being substituted by phenylalanine. For each of the ZR strains, two growth conditions (with or without DCMU in the medium) have been compared. The presence of the second mutation (at codon 219) leads to notable increase (20-fold) in resistance to DCMU, whereas the resistance to atrazine is only 2-fold. The presence of DCMU, as the selective agent, was responsible for an enhanced herbicide resistance, irrespective of the concentrations used. Substantial modifications in the rate of cell growth and O2 yields were observed when the maximal concentration (480 microM) of DCMU was used. These modifications were reversible on withdrawal of the DCMU. Thus the reversible adaptative modifications also adds to the mutational effect observed.