A low potential iron-sulfur cluster bound to membranes in blue-green algae.

A new low temperature EPR signal at g = 1.92 was found in photoreduced membrane fractions prepared from blue-green algae, Nostoc muscorum, Tolypothrix tenuis, Phormidium persicinum, and Anabaena variabilis. The signal also appeared when the sample was chemically reduced in the dark. Judging from the spectrum, the signal was probably due to the reduced form of a membrane-bound iron-sulfur cluster, tentatively designated as Cluster D. The midpoint potential of Cluster D was determined to be -0.27 volts. Possible roles of the cluster in electron transfer systems are also discussed.