Concerning the Application of the Q Cycle to Electron Transport in Cyanobacteria

Abstract

Abstract A working concept for the regulation of electron transport in oxygenic photosynthesis is that the electron transfer rate between the two photosystems, PSI and PSII, is governed by a ‘Q-cycle’ pathway operating in the electron transport chain which connects the two ‘reaction center’ complexes. The ‘Q-cycle’ concept was initially inferred from studies on mitochondrial electron transport. This concept has been assumed to be relevant to the electron transport pathways operating in oxygenic photosynthesis, with the majority of studies done on chloroplasts or thylakoid membranes The present study examines the existence and properties of a putative ‘Q-cycle’ in cyanobacteria. Light-induced spectral changes associated with cytochrome redox reactions in intact cells of the cyanobacterium Synechococcus sp. corresponded to the oxidation-reduction of cytochrome f. A correlated reduction of heme b6 was, however, not observed. The absence of significant cytochrome b reduction might be considered inconsistent with the set of electron transfer events associated conceptually with a ‘Q-cycle’ model of the electron transfer events in the chain. However, because heme b6 in the intact cyanobacteria is mostly reduced, it is not observable as a net electron acceptor of the plastoquinol or semiquinone formed by electron transfer from photosystem II. The redox environment of intact cyanobacteria in the dark resting state has an ambient potential sufficiently reducing that the ‘Q-cycle’ pathway for electron transport, well studied and characterized for function in isolated thylakoid membranes or chloroplasts, is not observed. This apparent quandary’ is a consequence of the reducing (negative potential) intracellular redox environment of cyanobacteria, which imposes a reduced state on the b-hemes, thereby preventing observation of their light-induced reduction.