A MoFe protein (ΔnifE Av1) with a purity of ～80% was purified from a nifE-deleted mutant of Azotobacter vinelandii DJ35. Compared with MoFe protein purified from wild-type strain OP (OP Av1), ΔnifE Av1 had the same subunits composition, and had immune reaction with antibody to OP Av1, but its relative mobility in anaerobic native polyacrylamide gel electrophoresis (PAGE) was a little larger than that of OP Av1. Metal analysis showed that Mo and Fe contents of ΔnifE Av1 both apparently decreased. When complemented with OP Fe protein, ΔnifE Av1 had no C2H2-reduction activity, but it could be in vitro activated by FeMoco extracted from OP Av1. The circular dichroism (CD) spectrum of ΔnifE Av1 at ~450 nm was similar to that of OP Av1, while the EPR signal at g≈3.7 was absolutely silent, and the signal intensities at g≈4.3 and 2.0 decreased by 75% and 50%, respectively. The results indicated that ΔnifE Av1 purified from DJ35 was a FeMoco-deficient but P-cluster-con- taining MoFe protein.