Crystal structure of a transcriptional activator of comK gene from Bacillus halodurans

ComK is the competence transcription factor of Bacillus subtilis whose synthesis activates the transcription of the competence genes.1 The expression of these genes induces competence, which is the ability to bind and take up exogenous DNA into the cell. The synthesis of ComK itself is regulated by a complex signal transduction network that involves a number of proteins.2 Med has been found as a transcriptional activator of the comK gene and the disruption of the med gene leads to a dramatic decrease in ComK levels in B. subtilis.3 The Berkeley Structural Genomics Center target 1957B (gi:10175511) is a 290 amino acid protein encoded by the med gene from Bacillus halodurans (hereafter referred to as BH1957). Its structure was unknown and no significant sequence homologues are found in the Protein Data Bank (PDB) structure database. Here, we report the crystal structure of BH1957 at 2.7 Å resolution determined by single wavelength anomalous diffraction (SAD) method. Analysis of the 3D structure of the protein shows that it consists of two similar a/b domains separated by a distinct cleft. Comparison with other protein structures reveals that BH1957 has, despite the absence of significant sequence identities, striking structural similarity to certain substrate-binding proteins and transcription regulators that contain the L-arabinose binding protein-like fold.