Structure of bacterial transcription factor SpoIIID and evidence for a novel mode of DNA binding

SpoIIID is evolutionarily conserved in endospore-forming bacteria, and it activates or represses many genes during sporulation of Bacillus subtilis. An SpoIIID monomer binds DNA with high affinity and moderate sequence specificity. In addition to a predicted helix-turn-helix motif, SpoIIID has a C-terminal basic region that contributes to DNA binding. The nuclear magnetic resonance (NMR) solution structure of SpoIIID in complex with DNA revealed that SpoIIID does indeed have a helix-turn-helix domain and that it has a novel C-terminal helical extension. Residues in both of these regions interact with DNA, based on the NMR data and on the effects on DNA binding in vitro of SpoIIID with single-alanine substitutions. These data, as well as sequence conservation in SpoIIID binding sites, were used for information-driven docking to model the SpoIIID-DNA complex. The modeling resulted in a single cluster of models in which the recognition helix of the helix-turn-helix domain interacts with the major groove of DNA, as expected. Interestingly, the C-terminal extension, which includes two helices connected by a kink, interacts with the adjacent minor groove of DNA in the models. This predicted novel mode of binding is proposed to explain how a monomer of SpoIIID achieves high-affinity DNA binding. Since SpoIIID is conserved only in endospore-forming bacteria, which include important pathogenic Bacilli and Clostridia, whose ability to sporulate contributes to their environmental persistence, the interaction of the C-terminal extension of SpoIIID with DNA is a potential target for development of sporulation inhibitors.