Crystallization and preliminary X-ray diffraction studies of two domains of a bilobed extra-cytoplasmic function sigma factor SigC from Mycobacterium tuberculosis.

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 (sigmaC) 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 approximately 25-amino-acid linker. The N-terminal domain contains the sigma2 DNA-binding motif, whereas the sigma4 motif is located in the C-terminal domain. Native sigmaC did not yield diffraction-quality crystals. However, two of its domains have been cloned, expressed and crystallized: sigmaC2 (12.3 kDa) and sigmaC4 (7.5 kDa). The sigmaC2 crystals belong to the hexagonal space group P6(1), with unit-cell parameters a = b = 85.28, c = 79.63 A, and native X-ray diffraction data were collected from this domain to 2.7 A on an in-house X-ray home source. The sigmaC4 crystals belong to the cubic space group F23, with unit-cell parameters a = b = c = 161.21 A. X-ray diffraction data were collected from this domain to 3.1 A, also on an in-house X-ray source.