Concentration and function of membrane-bound cytochromes in cyanobacterial heterocysts.

Membranes isolated from heterocysts and vegetative cells of Anabaena 7120 were assayed for content of cytochrome f, cytochrome b-563, cytochrome b-559(HP), cytochrome b-559(LP), and cytochrome aa(3) by use of reduced-minus-oxidized difference spectra. The level of cytochrome aa(3) in heterocyst membranes was 4 to 100 times higher than that in vegetative cells of Anabaena 7120 or other species of cyanobacteria. Heterocyst membranes lack cytochrome b-559(HP) but contain cytochrome b-559(LP) (E(m7.5) = +77 millivolts, n = 1) at approximately the same concentration as cytochrome f. The role of cytochrome b-559(LP) in the hydrogenase-dependent electron transfer pathway was investigated with the inhibitor 2-(n-heptyl)-4-hydroxyquinoline N-oxide which blocks electron flow from hydrogenase to acceptors reacting with the plastoquinone pool. Addition of inhibitor elicited no change in the reduction level of cytochrome b-559(LP) indicating that this cytochrome is not directly involved in this pathway.