This presumed domain is about is about 360 residues long. The function of this domain is unknown. It is found in some proteins that have two C-terminal CBS Pfam:PF00571 domains. There are also proteins that contain two inserted Fe4S domains near the C-terminal end of the domain. The protein Swiss:O26943 has been misannotated as an inosine monophosphate dehydrogenase based on the similarity to the CBS domains. Based on genetic analyses in the methanogen Methanosarcina acetivorans, this family is a key component of the metabolic network for sulfide assimilation and trafficking in methanogens. It is essential to a novel, O-acetylhomoserine sulfhydrylase-independent pathway for homocysteine biosynthesis, and may catalyse sulfur incorporation into the side chain of an as yet unidentified amino acid precursor [1]. The DUF39-CBS and DUF39-ferredoxin architectures repeatedly occur together in the genomes of methanogenic Archaea, suggesting they may be of diverged function. This is consistent with a phylogenetic reconstruction of the DUF39 family, which clearly distinguishes the CBS-associated and ferredoxin-associated DUF39s [1].

1: Novel proteins for homocysteine biosynthesis in anaerobic microorganisms. Rauch BJ, Gustafson A, Perona JJ; Mol Microbiol. 2014;94:1330-1342. PMID:25315403