Expression of the nifA gene of Herbaspirillum seropedicae: role of the NtrC and NifA binding sites and of the-24/-12 promoter element

The nifA promoter of Herbaspirillum seropedicae contains potential NtrC. Ni and IHF binding sites together with a -12/-24 sigma(N)-dependent promoter. This region has now been investigated by deletion mutagenesis for the effe ct of NtrC and NifA on the expression of a nifA::lacZ fusion. A 5' end to t he RNA was identified at position 641, 12 bp downstream from the -12/-24 pr omoter. Footprinting experiments showed that the G residues at positions -2 6 and -9 are hypermethylated, and that the region from -10 to +10 is partia lly melted under nitrogen-fixing conditions, confirming that this is the ac tive nifA promoter. In H. seropedicae nifA expression from the sigma(N)-dep endent promoter is repressed by fixed nitrogen but not by oxygen and is pro bably activated by the NtrC protein. NifA protein is apparently not essenti al for nifA expression but it can still bind the NifA upstream activating s equence.