This is an SH3 domain found in antirepressor proteins such as CarS from Myxococcus xanthus. CarS antirepressor recognizes and neutralizes its cognate repressors to turn on a photo-inducible promoter. CarS physically interacts with the MerR-type winged-helix DNA-binding domain of these repressors leading to activation of carB operon. Structural studies of CarS from M. Xanthus reveals a beta-barrel fold akin to that in SH3 domains. However, it diverges from the typical SH3 domain fold in the lengths and conformations of the connecting loops. Functional analysis reveal that SH3 domain-like fold in the antirepressor CasS, mimics operator DNA in sequestering the repressor DNA recognition helix to activate transcription [1].

1: A bacterial antirepressor with SH3 domain topology mimics operator DNA in sequestering the repressor DNA recognition helix. Leon E, Navarro-Aviles G, Santiveri CM, Flores-Flores C, Rico M, Gonzalez C, Murillo FJ, Elias-Arnanz M, Jimenez MA, Padmanabhan S; Nucleic Acids Res. 2010;38:5226-5241. PMID:20410074