This domain is found at the C-terminal end of DNA-binding protein BldD from Streptomyces venezuelae (Swiss:F2RCL8) and similar proteins found in Actinobacteria. This domain shows two beta-alpha- alpha repeats and a short C-terminal helix. It is responsible for the binding of the protein to the second messenger 3',5'-cyclic diguanylic acid (c-di-GMP) [1-3].

1: Tetrameric c-di-GMP mediates effective transcription factor dimerization to control Streptomyces development. Tschowri N, Schumacher MA, Schlimpert S, Chinnam NB, Findlay KC, Brennan RG, Buttner MJ; Cell. 2014;158:1136-1147. PMID:25171413

2: The Streptomyces master regulator BldD binds c-di-GMP sequentially to create a functional BldD2-(c-di-GMP)4 complex. Schumacher MA, Zeng W, Findlay KC, Buttner MJ, Brennan RG, Tschowri N; Nucleic Acids Res. 2017;45:6923-6933. PMID:28449057

3: The C-terminal domain of the transcriptional regulator BldD from Streptomyces coelicolor A3(2) constitutes a novel fold of winged-helix domains. Kim JM, Won HS, Kang SO; Proteins. 2014;82:1093-1098. PMID:24356916