Azotobacter vinelandii contains a prrF-like sequence in a noncoding region of the chromosome. Like the Pseudomonas aeruginosa PrrF small RNA-encoding genes, the expression of the sequence, herein named arrF (Azotobacter regulatory RNA involving Fe), was increased 100-fold in wild-type cells in response to iron depletion. The level of ArrF was also increased to the same degree in the iron-replete fur mutant, but down back to a wild-type level when this fur mutant was complemented with the wild-type fur gene. These results, with the location of arrF gene in a noncoding region, suggest that this gene encodes an iron-responsive small RNA whose expression is negatively regulated by the Fur-Fe2+ complex. Disruption of this arrF gene upregulated the expression of iron-containing superoxide dismutase and FeSII protein, whereas fur mutation or iron depletion decreased the level of their transcript. A short region in the 5'-untranslated region of each transcript was predicted to be quite complementary to the core sequence of ArrF, assuming that ArrF represses the expression of the genes under Fur control by an antisense RNA mechanism. However, unlike the P. aeruginosa PrrF that has extensive targets in the tricarboxylic acid cycle and glyoxylate cycle, ArrF had little effect on those genes. The findings that there is a poor overlap between ArrF and PrrF targets and that the FeSII gene, which is present only in the chromosome of nitrogen-fixing bacterial species, is controlled by ArrF suggest that ArrF might have unique targets, some of which are involved in N-2 fixation.