Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. This sigma factor is responsible for the expression of the nitrogen fixation genes. The open complex (sigma-54 and core RNA polymerase) serves as the receptor for receipt of the melting signal from the remotely bound activator protein NifA for the expression of the nitrogen fixation proteins. Belongs to the sigma-54 factor family.