WYL is a Sm-like SH3 beta-barrel fold containing domain. It is a member of the WYL-like superfamily, named for three conserved amino acids found in a subset of the superfamily. However, these residues are not strongly conserved throughout the family. Rather, the conservation pattern includes four basic residues and a position often occupied by a cysteine [1], which are predicted to line a ligand-binding groove typical of the Sm-like SH3 beta-barrels. A WYL domain protein (sll7009) is a negative regulator of the I-D CRISPR-Cas system in Synechocystis sp [2]. It is predicted to be a ligand-sensing domain that could bind negatively charged ligands, such as nucleotides or nucleic acid fragments, to regulate CRISPR-Cas and other defense systems such as the abortive infection AbiG system.

1: CARF and WYL domains: ligand-binding regulators of prokaryotic defense systems. Makarova KS, Anantharaman V, Grishin NV, Koonin EV, Aravind L; Front Genet. 2014;5:102. PMID:24817877

2: Adaptation and modification of three CRISPR loci in two closely related cyanobacteria. Hein S, Scholz I, Voss B, Hess WR; RNA Biol. 2013;10:852-864. PMID:23535141