The NifA protein is the central regulator of the nitrogen fixation genes.It activates transcription of nif genes by an alternative holoenzyme form of RNA polymerase containing the σ54 factor.The NifA protein from Klebsiella pneumoniae consists of the N-terminal domain of unknown function,the central catalytic domain with ATPase activity and the C-terminal DNA-binding domain.The Kp NifA protein is sensitive to temperature,while the Enterobacter cloacae NifA protein is less sensitive to temperature than Kp NifA.Our results show that the N-terminal domain of NifA plays the decisive role in the temperature sensitivity of the protein.