Structural insights into the dimerization of the response regulator ComE from Streptococcus pneumoniae

Natural transformation contributes to the maintenance and to the evolution of the bacterial genomes. In Streptococcus pneumoniae, this function is reached by achieving the competence state, which is under the control of the ComD-ComE two-component system. We present the crystal and solution structures of ComE. We mimicked the active and non-active states by using the phosphorylated mimetic ComE^D58E and the unphosphorylatable ComE^D58A mutants. In the crystal, full-length ComE^D58A dimerizes through its canonical REC receiver domain but with an atypical mode, which is also adopted by the isolated REC^D58A and REC^D58E. The LytTR domain adopts a tandem arrangement consistent with the two direct repeats of its promoters. However ComE^D58A is monomeric in solution, as seen by SAXS, by contrast to ComE^D58E that dimerizes. For both, a relative mobility between the two domains is assumed. Based on these results we propose two possible ways for activation of ComE by phosphorylation.