DCMU-type herbicides

The phenolic photoaffinity label azidodinoseb (Figure 4) binds less specifically than either azidoatrazine or azidotriazinone (14). In addition to other proteins, it labels predominantly the photosystem II reaction center proteins (spinach 43 and 47 kDa Chlamydomo-nas 47 and 51 kDa) (17). Because of the unspecific binding of azidodinoseb, this can best be seen in photosystem II preparations (17). Thus, the phenolic herbicides bind predominantly to the photosystem II reaction center, which might explain many of the differences observed between "DCMU-type" and phenolic herbicides (9). The photosystem II reaction center proteins and the 34 kDa herbicide binding protein must be located closely to and interact with each other in order to explain the mutual displacement of both types of herbicides (8,12,21). Furthermore, it should be noted that for phenolic herbicides, some effects at the donor side of photosystem II (22) and on carotenoid oxidation in the photosystem II reaction center have been found (23). 

As mentioned above, the mutation Tvr222- Phe in Rp. viridis results in an unexpected change in its behavior towards herbicides. Tvr222. being about 7 A away from ubiquinone, is not expected to be involved in the binding of either Qb or the herbicide molecule. Nevertheless, mutant T4 is now resistant to triazine herbicides but instead is sensitive toward DCMU, which binds to PS II but not to the wild-type bacterial reaction center. The interesting question is how does mutant T4 become DCMU-sensitive ... 

Fluorescence decay after a flash The kinetics of reoxidation monitors the fluorescence decay after a flash. The initial part of the decay was the same in herbicide resistant and susceptible Syneohooystis 6714 but the quasi stationary fluorescence level was higher in DCMU-II, DCMU-IIg and AzV. The same type of decays were obtained for resistant and susceptible thylakoids ot Cheno podium aVbim (Fig. 1) Fast initial decay, higher plateau level in the resistant thylakoids as compared to the wild type. 

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