Resumen:
In pneumococcus, a quorum-sensing system controls the
development competence at pH 7.8. The competence-stimulating
peptide (CSP) is a cell-density signal secreted by an ABC
transporter (ComAB), and its accumulation in the extracellular
space is sensed by a transmembrane histidin kinase (ComD).
Upon CSP-binding, ComD phosphorilates a response regulator
(ComE), activating the transcription of the competence genes.
We demonstrated that autolysis is triggered by acidic stress at
pH 5.6 and mediated by ComE. However, competence is
abolished at pH < 6.8, suggesting that ComE has another
function in the acid-induced lysis. Here, we studied the
connection between ComE and the quorum-sensing system
under acidic conditions. To block the CSP circuit, we constructed
the comA::km strain, which lysed at pH 5.6 as the wild-type strain
(WT), but the lysis of the comA::ery/ comE::km mutant was
inhibited, indicating ComE regulation by a quorum sensingindependent
pathway. To investigate the putative
phosphorylation state of ComE at pH 5.6, we generated the
hyperphosphorylating comDT233I mutant, which lysed more
rapidly than WT, but the lysis of the ComDT233I/ comE::km mutant
was not blocked at pH 5.6. These results suggest that ComE-P is
necessary for the acid-induced lysis, but if ComE is absent,
ComD could phosphorylate another protein (by a signal pathway
known as ?crosstalk?) to induce lysis.