Fig. 2: Cryo-EM structures of the M. tuberculosis EσB protomer and dimer (EσB)2.
a Cryo-EM map and fitted molecular model of EσB (protomer R1): view from the RNA exit channel (left) and from the main channel (right). The RNAP subunits are colored as indicated on the left. The key structural RNAP modules68 are indicated. b Location of σB in the structure of the R1-R5 dimer (EσB)2. The RNAP core is shown as a gray semi-transparent molecular surface. The σB subunits assigned to different protomers are colored in deep sky blue and coral. The solved \({\sigma }_{2}^{{{{{{\rm{B}}}}}}}\) domain is shown as a cartoon with cylindrical helices. The unmodeled C-terminal segments of σB (i.e. a1, a2, b1, b2) are shown as molecular surfaces. Green spheres indicate catalytic Mg2+ ions. c Steric clash between the C-terminal segment of σB and the RNA:DNA hybrid in the active site of RNAP from the R1-R5 dimer. The 5-nt nascent RNA (yellow) and 8-nt template DNA (pink) (from PDB ID 6KON) are shown as cartoons. The b2 density of σB is shown as a molecular surface colored sky blue. d Superposition of class I (blue) and class II (coral) R1-R5 dimers.
