This is an SH3 domain which can be found in diphtheria toxin repressor (DtxR) proteins. DtxR) from Corynebacterium diphtheriae regulates the expression of the gene on corynebacteriophages that encodes diphtheria toxin (DT). Other genes regulated by DtxR include those that encode proteins involved in siderophore-mediated iron uptake [1].

1: Structures of three diphtheria toxin repressor (DtxR) variants with decreased repressor activity. Pohl E, Goranson-Siekierke J, Choi MK, Roosild T, Holmes RK, Hol WG; Acta Crystallogr D Biol Crystallogr. 2001;57:619-627. PMID:11320302