The psbA gene of DCMU-resistant Euglena gracilis has an amino acid substitution at serine codon 265.

The psbA gene coding for the herbicide binding QB protein of photosystem II has been sequenced previously (Karabin et al. 1984). A herbicide resistant mutant of Euglena, Euglena gracilis ZR, was studied by sequencing part of the psbA gene and its corresponding mRNA. Sequencing reactions were done by annealing a psbA specific, end-labeled DNA-oligomer to total chloroplast DNA or RNA and extending this primer with reverse transcriptase in the presence of the four dideoxynucleotides. An amino acid substitution from serine to alanine at position 265 was detected. All known herbicide resistant higher plants sequenced to date and the Chlamydomonas mutant DCMU-4 show a change at exactly this same position, but the substitution in higher plants is from serine to glycine.