Multifunctional NadR proteins are involved in transcriptional regulation of nicotinamide adenine dinucleotide (NAD) biosynthesis and salvage genes in Enterobacteria [20857400, 16078372]. NadR proteins in E. coli and other Enterobacteria are composed of three domains, an N-terminal HTH DNA binding region (PF01381), central nicotinamide mononucleotide adenylyltransferase domain and C-terminal ribosylnicotinamide kinase domain . The latter two enzymatic domains are involved in the ribosylnicotinamide salvage pathway. Under nonstarvation conditions NadR is bound with its co-repressor, NAD(+), and represses expression of the target genes.
When concentration of co-repressor decreases, NadR binds ATP instead of NAD(+) and relieves transcription of the repressed genes [15968050
]. NadR orthologs are widely distributed in other bacterial linages, however the N-terminal regulatory domain is present only in Enterobacteria. Thus, the phylogenetic distribution of NadR regulon is limited to Enterobacteria, and microorganisms from other lineages use transcription factors from different protein families (NiaR, NrtR) to control NAD metabolism.