Cheng W, Roth J
Biology Department, University of Utah, Salt Lake City 84112, USA.
J Bacteriol. 1995 Dec;177(23):6711-7. doi: 10.1128/jb.177.23.6711-6717.1995.
The NAD or pyridine nucleotide cycle is the sequence of reactions involved in the breakdown of NAD to nicotinamide mononucleotide (NMN) and regeneration of NAD. This cycle is fivefold more active during aerobic growth of Salmonella typhimurium and under this condition breaks down half of the NAD pool every 90 min. DNA ligase is known to convert NAD to NMN but is only a minor contributor to the NAD cycle during aerobic growth. The dominant aerobic route of NMN formation is otherwise uncharacterized. Accumulated NMN generated by either of these routes is potentially dangerous in that it can inhibit the essential enzyme DNA ligase. The reactions which recycle NMN to NAD may serve to minimize the inhibition of ligase and other enzymes by accumulated NMN. The predominant recycling reaction in S. typhimurium appears to be NMN deamidase, which converts NMN directly to the biosynthetic intermediate nicotinic acid mononucleotide. Mutants defective in this recycling step were isolated and characterized. By starting with a ligase-deficient (lig mutant) parent strain that requires deamidase to assimilate exogenous NMN, two classes of mutants that are unable to grow on minimal NMN media were isolated. One class (pncC) maps at 83.7 min and shows only 2% of the wild-type levels of NMN deamidase. Under aerobic conditions, a lig+ allele allows a pncC mutant to grow on NMN and restores some deamidase activity. This growth ability and enzyme activity are not found in lig+ strains grown without oxygen. This suggests that the existence of a second NMN deamidase (pncL) dependent on ligase and stimulated during aerobic growth. The second class of mutants (pncD) gains a requirement for isoleucine plus valine with growth in the presence of exogenous NMN. We propose that pncD mutations reduce the activity of an ilv biosynthetic enzyme that is naturally sensitive to inhibition by NMN.
烟酰胺腺嘌呤二核苷酸（NAD）或吡啶核苷酸循环是指NAD分解为烟酰胺单核苷酸（NMN）以及NAD再生所涉及的一系列反应。在鼠伤寒沙门氏菌有氧生长期间，这个循环的活性要高五倍，在此条件下，每90分钟就会分解一半的NAD库。已知DNA连接酶可将NAD转化为NMN，但在有氧生长过程中，它对NAD循环的贡献较小。NMN形成的主要有氧途径在其他方面尚未得到表征。通过这两种途径产生的累积NMN可能具有潜在危险性，因为它会抑制关键酶DNA连接酶。将NMN再循环为NAD的反应可能有助于将累积的NMN对连接酶和其他酶的抑制作用降至最低。鼠伤寒沙门氏菌中主要的再循环反应似乎是NMN脱氨酶，它可将NMN直接转化为生物合成中间体烟酸单核苷酸。分离并鉴定了在这个再循环步骤中存在缺陷的突变体。从需要脱氨酶来同化外源NMN的连接酶缺陷型（lig突变体）亲本菌株开始，分离出了两类无法在最低NMN培养基上生长的突变体。一类（pncC）位于83.7分钟处，其NMN脱氨酶水平仅为野生型的2%。在有氧条件下，一个lig +等位基因使pncC突变体能够在NMN上生长，并恢复一些脱氨酶活性。在无氧条件下生长的lig +菌株中未发现这种生长能力和酶活性。这表明存在第二种依赖连接酶并在有氧生长期间受到刺激的NMN脱氨酶（pncL）。第二类突变体（pncD）在有外源NMN存在的情况下生长时，对异亮氨酸和缬氨酸产生了需求。我们推测pncD突变会降低一种对NMN抑制天然敏感的ilv生物合成酶的活性。
