An indispensable gene for NAD biosynthesis in Salmonella typhimurium

We have located the nadD locus between lip and leuS at 14 min on the Salmonella typhimurium chromosome, and we have shown it to be the structural gene for nicotinic acid mononucleotide adenylyltransferase. This is the first indispensable gene of pyridine nucleotide metabolism that has been identified. Mutants altered at this locus, isolated by their 6-aminonicotinamide resistance phenotype, accumulate abnormally large pools of nicotinic acid mononucleotide in vivo; many exhibit a temperature-sensitive lethal phenotype. Enzyme assays reveal markedly lower transferase activity in mutant extracts than in nadD+ extracts. The partial dominance of nadD mutants when placed in a nadD+/nadD diploid suggests that nicotinic acid mononucleotide adenylyltransferase is a multimeric enzyme.