Crystal Structure of SpoVT, the Final Modulator of Gene Expression during Spore Development in Bacillus subtilis

Endospore formation in Bacillus subtilis is orchestrated by five developmental sigma factors and further modulated by several auxiliary transcription factors. One of these, SpoVT, regulates forespore-specific σ^G-dependent genes and plays a key role in the final stages of spore formation. We have determined the crystal structure of the isolated C-terminal domain of SpoVT at 1.5 Å by experimental phasing techniques and used this model to solve the structure of the full-length SpoVT at 2.6 Å by molecular replacement. SpoVT is a tetramer that shows an overall significant distortion mediated by electrostatic interactions. Two monomers dimerize via the highly charged N-terminal domains to form swapped-hairpin β-barrels. These asymmetric dimers further tetramerize through the formation of mixed helix bundles between their C-terminal domains, which themselves fold as GAF (cGMP-specific and cGMP-stimulated phosphodiesterases, Anabaena adenylate cyclases, and Escherichia coli FhlA) domains. The combination of a swapped-hairpin β-barrel with a GAF domain represents a novel domain architecture in transcription factors. The occurrence of SpoVT homologs throughout Bacilli and Clostridia demonstrates the ancestral origin of this factor in sporulation.