Figure 1: Evolution of the RNPP family in Bacilli.

The presumed ancestor protein contains X TPR motifs and an inhibitor C-terminal end. The first step of this evolution scheme is the separation of the ancestral gene in two separate genes encoding, from 5’ to 3’, the quorum sensor (Rap) and an inhibitor signaling peptide (Phr). The second step is the acquisition of an HTH domain resulting in a bifunctional protein (NprR) controlled by a molecular switch (NprX) having an inhibitory (-) or activating (+) effect. The third step is the loss of 3 TPR motifs and leads to the PlcR-PapR quorum sensing system.