A new mutant of the psbA gene conferring resistance to 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) was obtained by selection of photomixotrophic tobacco (Nkotiana tabacum cv Samsun NN) cells. The 264th codon AGT (serine) in the wild psbA gene was changed to ACT (threonine) in these mutant tobacco cells. All other higher plants resistant to atrazine exhibit a change to GGT (glycine) in this codon. Measurements of Hill reaction activity and chlorophyll fluorescence showed that the threonine 264-containing plastoquinone serving as secondary stable electron acceptor of PSII (QB protein) had not only strong resistance to triazine-type herbicides but also moderate resistance to substituted urea-type herbicides. Threonine-type QB protein showed especially strong resistance to methoxylamino derivatives of the substituted urea herbicides. The projected secondary structures of the mutant Q5 proteins indicate that the crossresistance of threonine 264 QB protein to triazine and urea herbicides is mainly due to a conformational change of the binding site for the herbicides. However, the glycine 264 Os protein is resistant to only triazine herbicides because of the absence of an hydroxyl group and not because of a conformational change.Photosystem II (PSII) complex has the important function of splitting water to form molecular oxygen in photosynthesis. Among the components of the PSII complex, the 32 kD polypeptide (known as the QB3 or Dl protein) has a special importance since it forms part of the reaction center which transports electrons from the primary electron acceptor, QA, to the secondary stable electron acceptor, QB (20). The QB protein is also important as the primary target site of many herbicides which act by inhibiting photosynthesis (triazines, substituted ureas, triazinones, uracils and so on) (9, 24).The amino acid sequence ofthe QB protein was determined from the sequence ofthe psbA gene which encodes this protein (11,13, 33). The 3-dimensional structure of the QB protein, however, has not been determined since it is extremely unstable in the isolated state. Thus, the mechanisms of electron transport and herbicide-binding in PSII are not fully under-'