An 11-kbp DNA element of unknown function interrupts the nifD gene in vegetative cells of Anabaena sp. strain PCC 7120. In developing heterocysts the niJD element excises from the chromosome via site-specific recombination between short repeat sequences that flank the element. The nucleotide sequence of the niJp-proximal half of the element was determined to elucidate the genetic potential of the element. Four open reading frames with the same relative orientation as the nijD element-encoded xisA gene were identified in the sequenced region. Each of the open reading frames was preceded by a reasonable ribosome-binding site and had biased codon utilization preferences consistent with low levels of expression. Open reading frame 3 was highly homologous with three cytochrome P-450 w-hydroxylase proteins and showed regional homology to functionally significant domains common to the cytochrome P-450 superfamily. The sequence encoding open reading frame 2 was the most highly conserved portion of the sequenced region based on heterologous hybridization experiments with three genera of heterocystous cyanobacteria.Anabaena sp. strain PCC 7120 is an oxygen-evolving photoautotroph that is capable of simultaneous photosynthesis and nitrogen fixation. This strain is a well-studied representative of a class of filamentous cyanobacteria that protect nitrogenase from oxygen inactivation by sequestering the enzyme in specialized nitrogen-fixing cells called heterocysts.