Immunoblot of the dinitrogenase reductase in UR2 (wild-type), UR2501 (wild-type NifA in wild-type background), UR2502 (NifA-M173I in wild-type background), UR2503 (NifA-M173V in wild-type background), UR2327 (wild-type NifA in draT mutant background), UR2328 (NifA-M173I in draT mutant background) and UR2329 (NifA-M173V in draT mutant background) when cells were grown in MN (NH4+ -excess) medium. Proteins were extracted quickly with TCA, separated by SDS-PAGE, and immunoblotted with antibody against A. vinelandii dinitrogenase reductase. As described in Methods, active dinitrogenase reductase is unmodified and migrates as a single band (indicated by U), while inactive dinitrogenase reductase migrates as two bands with the upper band representing the modified subunit (indicated by M). A third band between the unmodified and the modified bands might be an alternative dinitrogenase reductase or another unmodified form of dinitrogenase reductase.