Abstract
The role of conserved nucleotides in nitrogen-fixation promoter function has been examined using both oligonucleotide and chemical mutagenesis to introduce base changes in the Klebsiella pneumoniae nifL and nifH promoters. Among ten mutations analysed, including six spontaneous mutations, base changes at -12, -13, -14, and -26, located in previously identified conserved sequences, perturbed the activity of the promoters, demonstrating that these sequences are required for transcription. Not all base changes produced similar strong promoter down phenotypes when the nifL and nifH promoters were compared: activation of the nifH promoter by the nifA gene product was less sensitive to base changes in conserved nucleotides than was activation of the equivalently altered nifL promoter by the nifA or ntrC products. We have found that the nifH promoter can be weakly activated by the ntrC product; this activation shows the same down response to base changes seen with ntrC activation of the nifL promoter. We present evidence that the efficient activation of the nifH promoter by nifA (but not ntrC) can be attributed to specific upstream sequences present in the nifH promoter.