This domain occurs as an additional C-terminal iron-sulfur cluster binding domain in many radical SAM domain, Pfam:PF04055 proteins. The domain occurs in a number of proteins that modify a protein to become an active enzyme, or a peptide to become a ribosomal natural product. The domain is named SPASM because it occurs in the maturases of Subilitosin, PQQ, Anaerobic Sulfatases, and Mycofactocin.

1: Biological systems discovery in silico: radical S-adenosylmethionine protein families and their target peptides for posttranslational modification. Haft DH, Basu MK; J Bacteriol. 2011;193:2745-2755. PMID:21478363