Nitric oxide (NO) inhibits respiration and induces a dormancy program in Mycobacterium tuberculosis. The study shows that low, nontoxic concentrations of NO and oxygen deprivation (hypoxia) induce a 48-gene regulon in M. tuberculosis, which prepares the bacteria for survival during long periods of dormancy. NO reversibly inhibits aerobic respiration and growth, while hypoxia leads to a more quiescent state. A heme-containing enzyme, possibly the terminal oxidase in the respiratory pathway, senses and integrates NO and oxygen levels, signaling the regulon. These findings suggest that within granulomas, NO production and oxygen limitation constrain M. tuberculosis replication in persons with latent tuberculosis. The dormancy regulon is regulated by the DosR system, which is essential for hypoxic dormancy. The study also shows that NO and hypoxia induce a common set of 48 genes, which are clustered in nine discrete modules. In vivo, these genes are expressed in infected tissue, indicating their role in the latent state. The dormancy regulon enhances the survival of M. tuberculosis during in vitro dormancy. NO inhibits respiration and growth, and its effects are reversible. The study also shows that NO and oxygen compete for sensing by a heme-containing protein, suggesting a role in the NO/low oxygen signal transduction system. The findings support a model where NO and oxygen levels control respiration, growth, and gene regulation in M. tuberculosis. The study highlights the importance of NO in the host's ability to control M. tuberculosis replication and maintain latency.Nitric oxide (NO) inhibits respiration and induces a dormancy program in Mycobacterium tuberculosis. The study shows that low, nontoxic concentrations of NO and oxygen deprivation (hypoxia) induce a 48-gene regulon in M. tuberculosis, which prepares the bacteria for survival during long periods of dormancy. NO reversibly inhibits aerobic respiration and growth, while hypoxia leads to a more quiescent state. A heme-containing enzyme, possibly the terminal oxidase in the respiratory pathway, senses and integrates NO and oxygen levels, signaling the regulon. These findings suggest that within granulomas, NO production and oxygen limitation constrain M. tuberculosis replication in persons with latent tuberculosis. The dormancy regulon is regulated by the DosR system, which is essential for hypoxic dormancy. The study also shows that NO and hypoxia induce a common set of 48 genes, which are clustered in nine discrete modules. In vivo, these genes are expressed in infected tissue, indicating their role in the latent state. The dormancy regulon enhances the survival of M. tuberculosis during in vitro dormancy. NO inhibits respiration and growth, and its effects are reversible. The study also shows that NO and oxygen compete for sensing by a heme-containing protein, suggesting a role in the NO/low oxygen signal transduction system. The findings support a model where NO and oxygen levels control respiration, growth, and gene regulation in M. tuberculosis. The study highlights the importance of NO in the host's ability to control M. tuberculosis replication and maintain latency.