In the murine model of infection, a Mycobacterium tuberculosis mce1 operon mutant elicits an aberrant granulomatous response, resulting in uncontrolled replication and failure to enter a persistent state. In this study, we demonstrate that the mce1 genes can be transcribed as a 13-gene polycistronic message encompassing Rv0166 to Rv0178. Quantitative reverse transcriptase PCR and immunoblot analyses revealed that the mce1 genes and proteins are expressed during in vitro growth but are significantly down-regulated in intracellular bacilli isolated from murine macrophages. A homologue of the FadR subfamily of GntR transcriptional regulators, Rv0165c (designated Mce1R), lies upstream and is divergently transcribed from the operon. To investigate whether this gene plays a role in regulation of mce1 expression, we created an M. tuberculosis mce1R deletion mutant. There was no difference in expression of mce1 operon genes in ⌬mce1R compared to expression in the wild type during logarithmic growth in vitro. However, in bacilli isolated from murine macrophages, expression of mce1 genes was significantly higher in ⌬mce1R. In addition, overexpression of mce1R resulted in repression of the mce1 genes. These data demonstrate that Mce1R is a negative regulator that acts intracellularly to repress expression of the mce1 operon. We propose that Mce1R facilitates balanced temporal expression of the mce1 products required for organized granuloma formation, which is both protective to the host and necessary for the persistence of M. tuberculosis.Tuberculosis causes approximately two million deaths annually, and it has been estimated that two billion people are currently infected with the causative agent, Mycobacterium tuberculosis (10). Infection is initiated by inhalation of the bacilli into the lung, where they are ultimately ingested by alveolar macrophages. In the typical immunocompetent host, the cellmediated immune response results in the formation of a granuloma, the pathological hallmark of tuberculosis. In the majority of cases, this response is sufficient to contain the infection and prevent disease; however, viable bacilli remain within granulomas and can cause active disease years later.In the past decade, dramatic advances facilitating the molecular biological manipulation of mycobacteria have increased our knowledge of the mechanisms that tubercle bacilli use to survive within macrophages (33); however, little remains known about the bacterial determinants of infection outcome. Previous studies suggest that the M. tuberculosis mce1 operon is important in establishing a persistent infection in mice (32). It was shown that M. tuberculosis mce1 mutants failed to induce a characteristic proinflammatory response in ex vivo-infected murine macrophages. This phenotype was manifested in vivo by aberrant inflammatory cell migration and poor granuloma formation, leading to uncontrolled bacterial growth and rapid death of mutant-infected mice.The six Mce1 proteins (Mce1A to Mce1F) contain putative signal sequences indicative o...

SummaryMycobacterium tuberculosis causes a variety of clinical outcomes determined by host as well as bacterial factors. M. tuberculosis disrupted in the mce1 operon causes increased mortality in immunocompetent mice. This operon is negatively regulated by mce1R (Rv0165c). We studied the role of mce1R in infection outcome in mice. At 5 ¥ 10 4 tail vein infectious dose, the median survival time (MST) of mice infected with the mce1R mutant M. tuberculosis H37Rv was 293 days, while mice infected with the wild-type H37Rv survived more than 350 days (P < 0.0001). At a higher dose (5 ¥ 10 6 ), the MST of mutant-infected mice was 32 days, compared with 127 days for wild type-infected mice (P < 0.0001). With either tail vein or aerosol infection, mutantinfected mice developed larger granulomatous lesions in their lungs than mice infected with the wild type. Mutant-infected mice were unable to control the bacterial burden in the first 4 weeks of infection, but even after achieving control later, these mice succumbed to granulomatous pneumonia. These observations suggest that the early deregulated expression of the mce1 operon products determines later granulomatous tissue response. mce1 operon may homeostatically regulate the cell wall architecture in vivo that elicits a steady-state granuloma tissue response permitting M. tuberculosis to establish a long-term infection.