Concentration and Function of Membrane-Bound Cytochromes in Cyanobacterial Heterocysts

Abstract

Membranes isolated from heterocysts and vegetative cells of Anabaena 7120 were assayed for content of cytochrome f, cytochrome b-563, cytochrome b-559HP, cytochrome b-559LP, and cytochrome aa3 by use of reduced-minus-oxidized difference spectra. The level of cytochrome aa3 in heterocyst membranes was 4-100 times higher than that in vegetative cells of Anabaena 7120 or other species of cyanobacteria. Heterocyst membranes lack cytochrome b-559HP but contain cytochrome b-559LP (Em7.5 [midpoint potential, pH = 7.5] = +77 millivolts, n = 1) at approximately the same concentration as cytochrome f. The role of cytochrome b-559LP 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-559LP, indicating that this cytochrome is not directly involved in this pathway.