Contact-independent killing mediated by a T6SS effector with intrinsic cell-entry properties

Abstract

Bacterial type VI secretion systems (T6SSs) inject toxic effectors into adjacent eukaryotic and prokaryotic cells. It is generally thought that this process requires physical contact between the two cells. Here, we provide evidence of contact-independent killing by a T6SS-secreted effector. We show that the pathogen Yersinia pseudotuberculosis uses a T6SS (T6SS-3) to secrete a nuclease effector that kills other bacteria in vitro and facilitates gut colonization in mice. The effector (Tce1) is a small protein that acts as a Ca2+- and Mg2+-dependent DNase, and its toxicity is inhibited by a cognate immunity protein, Tci1. As expected, T6SS-3 mediates canonical, contact-dependent killing by directly injecting Tce1 into adjacent cells. In addition, T6SS-3 also mediates killing of neighboring cells in the absence of cell-to-cell contact, by secreting Tce1 into the extracellular milieu. Efficient contact-independent entry of Tce1 into target cells requires proteins OmpF and BtuB in the outer membrane of target cells. The discovery of a contact-independent, long-range T6SS toxin delivery provides a new perspective for understanding the physiological roles of T6SS in competition. However, the mechanisms mediating contact-independent uptake of Tce1 by target cells remain unclear.

T6SS secretes an LPS-binding effector to recruit OMVs for exploitative competition and horizontal gene transfer

“…To the best of our knowledge, this study is the first to report the role of T6SS in bacterial competition through the use of OMVs. It further supports the idea that T6SS gives bacteria a competitive advantage and acts as evolutionary factors that shape the composition of the microbial community [43,[66][67][68].…”

Immunity proteins of dual nuclease T6SS effectors function as transcriptional repressors

“…Optical density (OD 600 ) of the bacterial cultures was measured at 2-h time intervals and growth curve was plotted (time interval on x-axis and OD 600 on y-axis). DAPI staining and microscopic analysis 4 0 ,6-diamidino-2-phenylindole (DAPI) staining and microscopic analysis were performed to visualize cell death and nuclear degradation in the recombinant E. coli cells, as described previously (Pissaridou et al, 2018;Song et al, 2021). Briefly, 10 ml of recombinant E. coli (BL21) cells were grown in LB media containing appropriate antibiotics.…”

Roles of Type VI Secretion System in Transport of Metal Ions

“…Traditionally, T6SS is recognized as a contact-dependent molecular machinery. Recent studies revealed that T6SSs play crucial roles in shaping the composition of a microbial population in hosts or environmental niches, either by directly killing competing cells via contactdependent (Russell et al, 2011) and contact-independent (Song et al, 2021) translocation of toxins, or by competing for essential nutrients via contact-independent secretion of metal ion binding effectors. For example, the T6SS-HmuR-mediated active zinc transport system is involved in a contact-independent bacteriabacteria competition for nutrients (Si et al, 2017a,b;Han et al, 2019;Wang et al, 2021).…”

“…It has been reported that the T6SS function is determined by the loading effectors that can be delivered extracellularly based on energy consumption and load transport (Cianfanelli et al, 2016;Coulthurst, 2019). Many T6SS effectors related to virulence or competition that target the cell wall, membranes, and nucleic acids have been reported (Yang et al, 2018;Song et al, 2021). Several effectors with special activities have also been found.…”