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 6 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-Sequencing (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 .