Fig. 1.

(A). Overview on chromosomal positions of nitrate-responsive genes. Chromosome numbers are indicated in Roman numerals and the scale bar represents 1 kb. The nitrate cluster is located on chromosome VIII and comprises nitrate reductase (niaD), nitrite reductase (niiA) and one nitrate/nitrite transporter (nrtA/crnA). The second nitrate/nitrite transporter (nrtB/crnB) is located on the same chromosome but separated from the nitrate cluster. Genes encoding the two flavohemoglobins fhbA and fhbB are located on chromosome IV and II, respectively. Arrows indicate direction of transcription. (B). Northern blot (upper panel) and densitometric analysis (lower panel) of niiA steady-state levels in A. nidulans wild type and mutant strains lacking nitrate-related functions. Strains deficient in either nitrate reductase (niaDΔ), both nitrate transporters (nrtA⁻ and nrtB⁻) or all three genes were grown in liquid GMM for 14 h with either 3 mM l-arginine (white bars) or 5 mM l-proline (shaded bars) as sole nitrogen source. For ${\text{NO}}_2^{-}$ induction cells (black bars) were grown on l-arginine and induced by addition of 10 mM ${\text{NO}}_2^{-}$ for 15 min. 18S rRNA was used as loading control and reference for densitometric analysis. ³²P signal intensities were recorded by phosphoimaging and expression levels were calculated relative to induced wild type transcript levels which were arbitrarily set to 1.