Lessons and questions from the structure of the Spo0A activation domain: Response

Dimer-induced signal propagation in Spo0A

Spo0A, the response regulator protein controlling the initiation of sporulation in Bacillus, has two distinct domains, an N-terminal phosphoacceptor (or receiver) domain and a C-terminal DNA-binding (or effector) domain. The phosphoacceptor domain mediates dimerization of Spo0A on phosphorylation. A comparison of the crystal structures of phosphorylated and unphosphorylated response regulators suggests a mechanism of activation in which structural changes originating at the phosphorylatable aspartate extend to the alpha4beta5alpha5 surface of the protein. In particular, the data show an important role in downstream signalling for a conserved aromatic residue (Phe-105 in Spo0A), the conformation of which alters upon phosphorylation. In this study, we have prepared a Phe-105 to Ala mutant to probe the contribution of this residue to Spo0A function. We have also made an alanine substitution of the neighbouring residue Tyr-104 that is absolutely conserved in the Spo0As of spore-forming Bacilli. The spo0A(Y104A) and spo0A(F105A) alleles severely impair sporulation in vivo. In vitro phosphorylation of the purified proteins by phosphoramidate is unaffected, but dimerization and DNA binding are abolished by the mutations. We have identified intragenic suppressor mutations of spo0A(F105A) and shown that these second-site mutations in the purified proteins restore phosphorylation-dependent dimer formation. Our data support a model in which dimerization and signal transduction between the two domains of Spo0A are mediated principally by the alpha4beta5alpha5 signalling surface in the receiver domain.

Two-vector assay as a tool for examining Spo0A gene transcription regulation

We have modified an assay using two compatible vectors that coexist in Escherichia coli cells and applied it in the investigation of the transcriptional activity of Spo0A, a key regulator of sporulation in Bacillus subtilis. We have chosen the promoters of the Spo0A dependent genes, spoIIE and spoIIA, involved in sporulation, in order to study the transcription activity solely of the DNA binding domain of Spo0A. We have prepared the two-vector system so that one vector contained the cloned C-Spo0A under the control of an inducible promoter, and the second vector (the promoter probe vector), was composed of the Spo0A dependent spoIIE and spoIIA promoters. Using this two-vector system in E. coli, we proved that C-Spo0A is able to interact with the E. coli transcription apparatus, recognizes both promoters and activates transcription from these promoters.

DNA complexed structure of the key transcription factor initiating development in sporulating bacteria

Sporulation in Bacillus species, the ultimate bacterial adaptive response, requires the precisely coordinated expression of a complex genetic pathway, and is initiated through the accumulation of the phosphorylated form of Spo0A, a pleiotropic response regulator transcription factor. Spo0A controls the transcription of several hundred genes in all spore-forming Bacilli including genes for sporulation and toxin regulation in pathogens such as Bacillus anthracis. The crystal structure of the effector domain of Spo0A from Bacillus subtilis in complex with its DNA target was determined. In the crystal lattice, two molecules form a tandem dimer upon binding to adjacent sites on DNA. The protein:protein and protein:DNA interfaces revealed in the crystal provide a basis for interpreting the transcription activation process and for the design of drugs to counter infections by these bacteria.