Cyclic Electron Flow Activity Research Articles

Endogenous cyclic electron transport in broken chloroplasts

Cyclic electron flow around photosystem I plays an important role in providing sufficient ATP for the photosynthetic assimilation of COz [ 1,2]. Cyclic activity may be observed in isolated intact chloroplasts [2,3] but is lost, along with NADP+ reduction, during the isolation of naked thylakoids; however, addition of ferredoxin has been reported [4,5] to restore NADP+ reduction and cyclic photophosphorylation (i.e. phosphorylation resulting from cyclic electron flow around photosystem I). This reconstituted cyclic phosphorylation was found to resemble cyclic phosphorylation in intact chloroplasts in being inhibited by antimycin (a specific inhibitor of cyclic electron flow [1,6,7]), by the quinone-analogue DBMIB [6,8] or by poor redox poising [ 1,5,6,9,10]. The reconstituted cyclic system has special value in studies of the cyclic pathway because there is no permeability barrier isolating substrates, inhibitors and membrane probes from the outer thylakoid surface. It has, however, the disadvantage of requiring large amounts of exogenous ferredoxin while still not giving restoration of cyclic electron flow at the rates seen in intact chloroplasts. An improved procedure is reported here that gives envelope-free thylakoids retaining almost full activity of cyclic electron flow. A spectroscopic characteristic of intact chloroplasts is the slow (milliseconds rise time) electrochromic