Isolated heterocysts of the N
2
-fixing
Anabaena cylindrica
, prepared by a combination of lysozyme and Yeda press treatments, are metabolically active with over 90% of the measurable nitrogenase activity being located in the heterocyst preparations after disruption of the intact filaments. The photosynthetic activities of such isolated heterocysts are characterized by an inability to carry out the photolysis of water or to fix CO
2
. The lack of O
2
evolution appears to be due in part to the depletion during heterocyst differentiation of Mn, a central component of the photosystem II reaction centre in O
2
-evolving algae. There is evidence that components of the photosynthetic electron transport chain on the reducing side of the photosystem II reaction centre are present and functional in heterocysts. These include cytochrome c
554
, plastocyanin, plastoquinone, cytochrome b
559
, P700, cytochrome b
563
, and iron-sulphur proteins which appear to correspond to centre A and centre B of higher plant chloroplasts. Soluble, or loosely bound ferredoxin is also present and involved in electron transport from ferredoxin to NADP. Isolated heterocysts photoreduce methylviologen when reduced 2,6-dichlorophenolindophenol and diphenylcarbazide serve as electron donors. They show P700 photo-oxidation and photoreduction, photosynthetic electron transport which is inhibited by 2,5-dibromo-3-methyl-6-isopropyl-
p
-benzoquinone an antagonist of plastoquinone, photophosphorylation, oxidative phosphorylation and ferredoxin-NADP oxido-reductase mediated reactions. The photosynthetic modifications of the heterocyst are such that electron transport and the generation of ATP for nitrogenase can occur without concomitant O
2
evolution and without nitrogenase having to compete with CO
2
fixation for ATP and reductant.
