Modeling the ComD/ComE/comcde interaction network using small angle X-ray scattering

The ComD-ComE two-component system controls the competence state of Streptococcus pneumoniae via the phospho-regulation of ComE, which fluctuates between monomeric and dimeric states. We previously showed that the non-phosphorylatable ComE(D) (58A) mutant is monomeric in solution, whereas the ComE(D) (58E) active mimic mutant dimerizes via its REC domains. The crystal structure of ComE(D) (58A) revealed an asymmetric dimer that may represent the activated form of ComE. Here, we investigated the binding between the catalytic domain of ComD, ComE and the promoter region comcde, using small angle X-ray scattering. ComD(catdom) is a dimer that adapts two monomers of ComE, one on each side, placing (Com) (E) D58 residue in front of (Com) (D) H248, a location that is convenient for the intermolecular transfer reaction of the phosphoryl group. The LytTR, ComE(D) (58A) and ComE(D) (58E) complexed with comcde are composed of two protein molecules per DNA duplex. Modeling the complexes against small angle X-ray scattering data indicated that ComE(D) (58E) bound to comcde forms a compact dimer similar to the crystal structure, whereas ComE(D) (58A) -comcde adopts more than one conformation with or without dimer contacts. The various oligomeric states of ComE induce different bending angles of the promoter, which provides a mechanistic scenario for the activation of ComE: the phosphorylation of ComE forces additional bending of comcde, and the release of this bending strain on DNA via the disruption of the ComE dimer may signal the shut-off of the competence state. DATABASE: The molecular models and experimental SAXS data have been deposited on SASBDB (Small Angle Scattering Biological Data Bank) (see http://www.sasbdb.org/aboutSASBDB/) under the SAS codes SASDAA7, SASDAB7 and SASDAC7.