PNNL

Abstract

effector proteins from the bacterium to a host cell; however, the secretion signal is poorly defined. Effector N-termini are thought to contain the signal, but they lack homology, possess no identifiable motif, and adopt intrinsically disordered structures. We identified a panel of RNA secretion signals that facilitated reporter translocation into host cells via a mechanism dependent upon the RNA chaperone Hfq. Each of these signals was localized to an RNA leader sequence preceding the translational start codon. To obtain this panel of RNA signals, we fused untranslated leader sequences from 42 different Salmonella effector proteins to the adenylate cyclase reporter (CyaA'), and tested each of them for translocation into J774 macrophages. RNA sequences derived from five effectors, gtgA, cigR, gogB, sseL, and steD were sufficient for CyaA' injection into host cells. The gtgA RNA also directed translocation of the ß-lactamase reporter. To determine the mechanism of signal recognition, we identified proteins that bound specifically to the gtgA RNA. One of the unique proteins identified was Hfq. Translocation of all five UTR fusions was abolished in the Hfq mutant, confirming the importance of Hfq. Our results suggest that Hfq may direct a subset of RNA transcripts to the T3S apparatus for translation and secretion. Signal diversity may explain why the T3S signal has been difficult to define.