FIG. 1.

Physical map of the nos gene region and kinetics of the nosD operon transcription. (A) Physical map and transcriptional organization. The wavy lines indicate transcripts from the respective promoters identified in this work or previously for nosR (43) and nosZ (10). Promoters are represented by triangles and labeled P with a qualifying subscript. The open bars show the locations and sizes of probes used in Northern hybridization. (B) Time-resolved appearance of the nosD transcript on shift of an aerobic culture to nitrate denitrification. Strain MK21 (representing wild-type traits) was grown aerobically (shaking frequency, 240 rpm) and probed for a nosD transcript by Northern blot analysis (+O₂). The culture was induced for denitrification by lowering the shaking frequency to 120 rpm (shift to low pO₂) and adding 1 g of sodium nitrate per liter (−O₂, +NO₃⁻). The transcript was followed for 1 h after the downshift. The 0.6-kb nosD probe, isolation of mRNA, and conditions for hybridization were as specified in Material and Methods.