Detalle de: TtsI: Beyond Type III Secretion System Activation in Rhizobia

The expression of the rhizobial symbiotic genes is controlled by various transcriptional regulators. After induction with appropriate plant flavonoids, NodD is responsible for the activation of the expression of genes related to Nod factor synthesis and secretion, but also, in most rhizobia harbouring a symbiotic type III secretion system (T3SS), the expression of ttsI. The ttsI gene encodes the positive regulator of the expression of T3SS-related genes, including those coding for structural components and for type III-secreted effector proteins. However, besides this general role among T3SS-harbouring rhizobia, different works have shown additional functions of TtsI in the regulation (positive or negative) of other bacterial traits such as the production of modified lipopolysaccharides or different types of motility (swimming or surface spreading). Interestingly, these additional functions appear to be rather specific than general among rhizobia. Moreover, in Sinorhizobium fredii HH103, TtsI affects the expression of various genes belonging to the nod regulon, including several transcriptional regulators. This review summarizes all the well-known bacterial traits affected by TtsI and describes other rhizobial genes that are regulated by TtsI but whose function remains to be established.