Gene organization of the nifHDK locus. The restriction map of the nifHDK operon in Azoarcus sp. strain BH72 shows nif gene-containing clones from a genomic library above and the construction of the fdxN deletion (pEN9ΔF) below the map. The bold arrow indicates the promoter region, and the light arrow shows the position of fdxN in plasmid pEN94. Sequences with inverted repeats for stem-loop formation are indicated by hairpins below the map. S, SalI; E, EcoRI; P, PstI; X, XhoI; K, KpnI; Bs, BsmI; Sm, SmaI; B, BamHI.