Quorum sensing is a cell-to-cell communication process that allows bacteria to
measure their population numbers and to synchronously alter gene expression in
response to changes in cell population density. At the core of the Vibrio
cholerae quorum-sensing signal transduction pathway lie four redundant small RNAs
(sRNAs), named the Quorum Regulatory RNAs (Qrr1-4). Expression of qrr1-4 is cell
population density-dependent due to a requirement for the quorum-sensing
controlled phosphorylated response regulator LuxO-P, which is abundant only at
low cell population density. When expressed, Qrr1-4 repress translation of HapR,
the "master" quorum-sensing transcription factor. Here we show a negative
feedback loop in which HapR activates transcription of the qrr genes, which
indirectly leads to hapR repression. Efficient feedback activation of the qrr
genes requires the simultaneous presence of LuxO-P (present only at low cell
population density) and HapR (present only at high cell population density). For
this reason, the feedback loop does not influence quorum sensing at steady-state
low or high cell population density. However, LuxO-P and HapR are simultaneously
present immediately following the switch from high to low cell density
conditions. In this state, the HapR feedback loop dramatically accelerates V.
cholerae's transition from the high to the low cell density mode.