The site of inhibition of DCMU against photosystem II in the cyanobacterium (Blue-green alga) Anabaena cylindrica was examined in electron transport and fluorescence studies. Isolated thylakoids catalyzed silicomolybdate photoreduction using H2O as electron donor; the steady-state reaction was completely inhibited by DCMU. This reaction is insensitive to DCMU in chloroplasts, since silicomolybdate accepts electrons from the primary photosystem II acceptor, and thus before the site of action of DCMU in higher plants. DCMU did not increase the steady-state level of fluorescence by inact A. cylindrica, nor affect the monophasic fluorescence induction, with or without dithionite, in contrast to the DCMU-dependent stimulation of steady-state fluorescence and replacement of rapid initial transients by a monophasic rise in Chlorella. Fluorescence by isolated cyanobacterial thylakoids was lowered by the electron acceptors silicomolybdate, p- benzoquinone, ferricyanide and anthraquinone-2-sulfonate, and not restored by DCMU addition, in contrast to results obtained with tobacco chloroplasts. Together with previous Findings that di- phenylcarbazide-dependent photosystem II activity of A. cylindrica thylakoids is insensitive to DCMU. The data indicate that the principal site of action of the inhibitor lies on the O2-evolving side of photosystem II in this cyanobacterium.
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Codd et al. (1980) studied this question.