Abstract
Cyanobacterin, a secondary metabolite produced by the cysnobacterium, Scytonema hofmanni, inhibits electron transport at a site in photosystem II. It was previously shown that a DCMU-resistant mutant of A. nidulans R2 was still susceptible to cyanobacterin (Gleason et al., Plant Science, 46 (1986) 5–10). Apparently, cyanobacterin acts at a site different from that of DCMU and similar PS II inhibitors. To confirm this conclusion, a cyanobacterin-resistant strain of A. nidulans R2 was produced by nitrosoguanidine mutagenesis and selected by growth in the presence of 4.7 μM cyanobacterin. Hill activity in mutant thylakoids was compared to that of the wild type membranes in the presence of ferricyanide and silicomolybdate as electron acceptors. Photosynthetic electron transport in the mutant membranes shows a high degree of resistance to cyanobacterin in both reactions. In contrast, the mutant exhibits the same susceptibility to DCMU inhibition as the wild type R2. Cyanobacterin acts at a unique site, inhibiting electron flow from quinone-A to quinone-B.
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