CHARACTERISTICS OF CYCLIC ELECTRON-TRANSPORT IN THE CYANOBACTERIUM PHORMIDIUM LAMINOSUM

Cyclic photophosphorylation and ferredoxin-plastoquinone oxidoreductas e (FQR) activity have been measured for the first time in the cyanobac terium Phormidium laminosum. They were found to be insensitive to inhi bition by up to 10 mu M antimycin, as well as to the similarly acting compound, J820. This is in contrast to results from pea thylakoids, bu t consistent with the two other known studies in cyanobacteria and ind icates differences between the higher plant and cyanobacterial process es at the level of FQR. The novel inhibitor HQNOBr affected cyclic pho tophosphorylation at similar concentrations in the two species, but it s mode of action differed. In FQR assays with peas, 40 mu M HQNOBr had effects consistent with inhibition of the cytochrome bf complex, wher eas in P. laminosum the inhibition appeared to be specific to FQR acti vity. Additional evidence for the specificity of HQNOBr as a cyanobact erial FQR inhibitor comes from the fact that it has no effects on flas h-induced absorbance changes at wavelengths characteristic for P700, c ytochromes b and f and plastocyanin. This is in contrast to the higher -plant bf complex inhibitor MOA-Stilbene which does have effects on fl ash-induced absorbance changes which are consistent with inhibition of bf complex activity. The data presented indicate that the cyanobacter ial and higher-plant cyclic electron transfer processes are similar in over all mechanism but differ in detail, both at the level of FQR act ivity and the cytochrome bf complex.