Summary: Anoxygenic photosynthetic electron transport from sulphide, culminating in either H evolution or CO photoassimilation, was shown to include the segment from plastoquinone to ferredoxin in the cyanobacterium Both sulphide-dependent H evolution and CO photoassimilation were inhibited by plastoquinone analogues. In the former reaction, the block was bypassed by reduced -tetramethyl--phenylenediamine (TMPD). The link between this segment of electron transport and the hydrogenase enzyme was shown to limit the rate of sulphide-dependent H evolution. The rate of flow of electrons through this pathway was lower than would be expected either from the amounts of available enzyme, as measured by the oxidation of reduced methyl viologen, or from rates of electron transport to CO photoassimilation. When the strong reductant sodium dithionite was added to intact cells, the resulting low redox potential significantly improved photosynthetic sulphide-dependent H evolution. Hydrogenase activity in cell free extracts was similarly affected by dithionite. It is suggested that ambient redox potential controls electron flow through the hydrogenase, so that surplus reducing power is removed via H evolution.


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