Bacteria necessitate multiple signal transduction systems to sense the ever-changing environments and mediate the cellular response accordingly. The major bacterial signal transduction systems are one-component system (1CS), two-component system (2CS) and extracytoplasmic function (ECF) \u03c3 factor. Compared to 1CSs and 2CSs, ECF \u03c3 factors have only been identified much later and therefore the knowledge about their molecular mechanisms and physiological roles is less profound. This thesis mainly focuses on the study of ECF \u03c3 factors from the bacterial phyla, Planctomycetes and Actinobacteria.