Mo nitrogenase consists of two component proteins: the Fe protein, which contains a [Fe4S4] cluster, and the MoFe protein, which contains two different classes of metal cluster: P-cluster ([Fe8S7]) and FeMoco ([MoFe7S9C homocitrate]). The P-cluster is believed to mediate the electron transfer between the Fe protein and the MoFe protein via interconversions between its various oxidation states, such as the all-ferrous state (P-N) and the one- (P+) and two-electron (P2+) oxidized states. While the structural and electronic properties of P-N and P2+ states have been well characterized, little is known about the electronic structure of the P+ state. Here, a mutant strain of Azotobacter vinelandii (DJ1193) was used to facilitate the characterization of the P+ state of P-cluster. This strain expresses a MoFe protein variant (designated Delta nifB beta-188(Cys) MoFe protein) that accumulates the P+ form of P-cluster in the resting state. Magnetic circular dichroism (MCD) spectrum of the P-cluster in the oxidized Delta nifB beta-188(Cys) MoFe protein closely resembles that of the P2+ state in the oxidized wild-type MoFe protein, except for the absence of a major charge-transfer band centered at 823 nm. Moreover,. magnetization curves of Delta nifB beta-188(Cys) and wild-type MoFe proteins suggest that the P2+ species in both proteins have the same spin state. MCD spectrum of the P+ state in the Delta nifB beta-188(Cys) MoFe protein, on the other hand, is associated with a classic [Fe4S4](+) cluster, suggesting that the P-cluster could be viewed as two coupled 4Fe clusters and that it could donate either one or two electrons to FeMoco by using one or both of its 4Fe halves. Such a mode of action of P-cluster could provide energetic and kinetic advantages to nitrogenase in the complex mechanism of N-2 reduction.