GlnR is the global transcriptional regulator of nitrogen assimilation in Streptomyces coelicolor. Under nitrogen starvation, GlnR controls the transcription of at least nine genes associated with nitrogen metabolism. In this study, we identified a new GlnR target gene, SCO2958, named nnaR (nitrate/nitrite assimilation regulator). In silico analysis of NnaR revealed the presence of two distinct domains: an N-terminal uroporphyrinogen-III synthase (HemD)-like enzymatic domain and a C-terminal DNA binding domain. Complementation experiments with a haemin auxotroph Escherichia coli ΔhemD mutant strain revealed that NnaR has no HemD activity. Physiological studies of an S. coelicolor nnaR : : Tn5062 mutant showed that NnaR is involved in regulating nitrite reduction. By electrophoretic mobility shift assays the functionality of the NnaR DNA binding domain was confirmed, and it was found that NnaR binds in front of the genes narK (putative nitrate extrusion protein), nirB (nitrite reductase), nirA (putative nitrite/sulphite reductase) and nasA (putative nitrate reductase), which are associated with nitrate/nitrite assimilation. Furthermore, a cooperative binding of NnaR together with GlnR to the nirB promoter was observed, suggesting that NnaR may act as a GlnR co-activator.