The FixJN receiver domain was understood until now as a molecular switch modulating both the activity of FixJC and the quaternary structure of FixJ, whereas the transcriptional activation function on nifA was assigned to the FixJC output domain. In the present study however, we identified yet another function for the FixJN receiver domain since it was found to be also specifically required for full activation of the fixK promoter. Indeed alanine scanning showed the involvement of FixJN strand (32 (Lys-31 and Gln-34) for activation of the fixK promoter. More specifically, mutation of these residues affected the recruitment of RNA polymerase at the fixK promoter without affecting FixJ~P DNA binding nor the recruitment of RNA polymerase at the nifA promoter. These results point to a specific contact between FixJ~P and RNA polymerase in the fixK preinitiation complex,
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