FIG. 4.

Location and function of NnrR in the FixLJ-FixK₂ regulatory cascade of B. japonicum. Details are explained in the text. Note that even though the model suggests a direct hierarchical organization of FixLJ, FixK₂, NnrR, and denitrification genes, additional control levels situated in between cannot be excluded. Similarly, N oxide signaling to NnrR may be direct or indirect. The dashed arrow refers to anaerobic activation of nirK (but not of norCBQD) by FixK₂, which is retained in an nnrR mutant background. Filled boxes symbolize putative binding sites for FixK₂ and NnrR which are similar to that of the consensus FNR box (TTGAT-N₄-ATCAA).