Signaling diversity and evolution of extracytoplasmic function (ECF) σ factors

► ECF σ factors are the third most abundant signaling principle in bacteria. ► Bacteria harbor 6 ECF σ factors on an average, with some genomes encoding over 100. ► ECF σ factors are regulated by an ever increasing number of diverse mechanisms. ► The current ECF classification defines more than 50 distinct conserved ECF groups. ► The classification allows functional predictions of (novel) signaling mechanisms.