Quorum sensing orchestrates parallel cell death pathways in via Type 6 secretion dependent and independent mechanisms.

Quorum sensing (QS) is a cell-to-cell communication process that enables bacteria to coordinate group behaviors. In colonies, a program of spatial-temporal cell death is among the QS-controlled traits. Cell death occurs in two phases, first along the colony rim, and subsequently, at the colony center. Both cell death phases are driven by the type VI secretion system (T6SS). Here, we show that HapR, the master QS regulator, does not control gene expression nor T6SS-mediated killing activity. Nonetheless, a Δ strain displays no cell death at the colony rim. RNA-Seq analyses reveal that HapR activates expression of an operon containing four genes of unknown function, Epistasis and overexpression studies show that two of the genes, and , are required to drive cell death in both a Δ and a Δ Δ strain. Thus, - are regulated by HapR but act independently of the T6SS machinery to cause cell death, suggesting that a second, parallel pathway to cell death exists in .