The promoter of nitrogen-regulated transport, argtr, has been mutationally altered in order to determine the features that are essential for its response to nitrogen availability. Deletions of all sequences upstream of position -44 or downstream of position +2 had no effect on nitrogen regulation of argTr. These deletions define a small region of 44 bp where all necessary features for nitrogen regulation are located. This region includes sequences highly homologous to the nif consensus promoter. Alteration of this particular sequence caused drastic changes in the response to changes of nitrogen availability, thus indicating that they are directly involved in regulation. This implies that the NtrC protein must also act within this small region of the promoter. The data are discussed in terms of current-hypotheses concerning nitrogen regulation. In addition, we have shown 1. that carbon regulation at this promoter must occur at a site upstream from the nitrogen promoter; 2. that nifA can replace ntrC in the regulation of argTr.