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Sigma factors are transcription-regulatory proteins that bind to RNA polymerase and facilitate promoter recognition. The so-called extracytoplasmic function sigma factors help a bacterium to respond to environmental conditions. Mycobacterium tuberculosis SigC (σC) is an extracytoplasmic sigma factor that is essential for lethality in a mouse model of infection and is conserved in all pathogenic mycobacterial species. This protein consists of two domains that are connected by an ∼25-amino-acid linker. The N-terminal domain contains the σ2 DNA-binding motif, whereas the σ4 motif is located in the C-terminal domain. Native σC did not yield diffraction-quality crystals. However, two of its domains have been cloned, expressed and crystallized: \sigma^{\rm C}_{2} (12.3 kDa) and \sigma^{\rm C}_{4} (7.5 kDa). The \sigma^{\rm C}_{2} crystals belong to the hexagonal space group P61, with unit-cell parameters a = b = 85.28, c = 79.63 Å, and native X-ray diffraction data were collected from this domain to 2.7 Å on an in-house X-ray home source. The \sigma^{\rm C}_{4} crystals belong to the cubic space group F23, with unit-cell parameters a = b = c = 161.21 Å. X-ray diffraction data were collected from this domain to 3.1 Å, also on an in-house X-ray source.

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