In Gram-positive Streptomyces coelicolor A3(2), SigR (Sc sigma(R)) of the group IV ECF sigma factor singly activates expression of more than 30 oxidation responsive genes. Of the two promoter-binding domains individually called region 2 and region 4 - within Sc sigma(R) all, we hereby report a 2.6 angstrom resolution structure of the -35 element interacting carboxyl-terminal region 4 (Sc sigma(R)(4)). Structural comparison of Sc sigma(R)(4) with the Escherichia coli SigE (Ec sigma(E)) in complex with Ec sigma(E) -35 element suggested that a single residue (Sc sigma(R) Met188 and Ec sigma(E) Arg171) may be responsible for distinguishing the one-base pair difference of the -35 elements - Sc sigma(R) (-31')ATTCC(-35') ((-31')A) vs. Ec sigma(E-31')GTTCC-35' ((-31')G) - by interacting with the -31'-base Further studies using expressed Sc sigma(R) indicate that the wild-type Sc sigma(R) with Met188 selectively interacted with the (-31')A sequence over the (-31')G sequence, whereas a mutation of Met188 to arginine resulted in interaction with both (-31')A and (-31')G sequences. Hence, we conclude that Met188 of Sc sigma(R) confers the (-31')A-selectivity in -35 element interaction by disfavoured interaction with the (-31')G base. (C) 2016 Elsevier Inc. All rights reserved.