This domain, with its characteristic highly conserved CSS sequence motif, is found N-terminal to the EAL (Pfam:PF00563) domain in many cyclic diguanylate phosphodiesterases, such as PdeB/C/D/G/N from E. coli K-12 [1]. It has two highly conserved cysteine residues with potential disulfide bond formation, which might control the enzymatic activity of the cytoplasmic C-terminal EAL domain [1].

1: Transmembrane redox control and proteolysis of PdeC, a novel type of c-di-GMP phosphodiesterase. Herbst S, Lorkowski M, Sarenko O, Nguyen TKL, Jaenicke T, Hengge R; EMBO J. 2018; [Epub ahead of print] PMID:29514851