Physical map of the putative nitrogen fixation ( nif ) operons in D. hafniense DCB-2. The nifH homologs are colored black and the homologs for nifD or nifK are colored orange. Genes involved in MoFe cofactor biosynthesis are colored green; note that nifK, nifE and nifN are also involved in the synthesis of MoFe cofactor. ABC-type transporter genes in the operons are colored blue. The nif operon II and IV that were induced in transcription by NO3- and O2, respectively, are indicated with arrows. PII; nitrogen regulatory protein-encoding gene, araC-like; AraC-type transcriptional regulator-encoding gene.