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 (sigma(C)) 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 similar to 25-amino-acid linker. The N-terminal domain contains the sigma(2) DNA-binding motif, whereas the sigma(4) motif is located in the C-terminal domain. Native sigma(C) did not yield diffraction-quality crystals. However, two of its domains have been cloned, expressed and crystallized: sigma(C)(2) (12.3 kDa) and sigma(C)(4) (7.5 kDa). The sigma(C)(2) crystals belong to the hexagonal space group P6(1), with unit-cell parameters a = b = 85.28, c = 79.63 angstrom, and native X-ray diffraction data were collected from this domain to 2.7 angstrom on an in-house X-ray home source. The sigma(C)(4) crystals belong to the cubic space group F23, with unit-cell parameters a = b = c = 161.21 angstrom. X-ray diffraction data were collected from this domain to 3.1 angstrom, also on an in- house X-ray source.