Interbacterial competition mediated by the type VIIb secretion system

Abstract

Successful occupancy of a given niche requires the colonising bacteria to interact extensively with the biotic and abiotic environment, including other resident microbes. Bacteria have evolved a range of protein secretion machines for this purpose with eleven such systems identified to date. The type VIIb secretion system (T7SSb) is utilised by Bacillota to secrete a range of protein substrates, including antibacterial toxins targeting closely related strains, and the system as a whole has been implicated in a range of activities such as iron acquisition, intercellular signalling, host colonisation and virulence. This review covers the components and secretion mechanism of the T7SSb, the substrates of these systems and their roles in Gram-positive bacteria, with a focus on interbacterial competition.

Keywords: T7SS; bacterial competition; contact-dependent inhibition; protein secretion; toxin; type VII secretion system.

Fig. 1.

Structural arrangement of the T7SSb. (a-f) Schematic representations and experimentally-determined structures of core components of the T7SSb. (a) Crystal structure of the S. aureus EsxA homodimer (2VRZ) [26]. The two subunits (pink and white) assemble in a head-to-tail arrangement. (b) EssC encompasses two N-terminal forkhead-associated (FHA) domains, two transmembrane helices and four C-terminal ATPase domains, D0 – D3. Crystal structures have been determined for the N-terminal FHA domains (5FWH) and the C-terminal ATPase D2 and D3 domains (5FV0) from G. thermodenitrificans [28]. (c) Crystal structure of the B. subtilis EsaB homologue YukD (2BPS) [44]. (d) EsaA homologues have a variable number of transmembrane helices; shown here are the S. aureus and S. gallolyticus homologues. The crystal structure of part of the S. gallolyticus extracellular domain is shown (7JQE) [45]. (e) No structural information is available for EssA however its predicted topology shows a single transmembrane helix with N-terminal extracellular domain [51]. (f) Crystal structure of the B. subtilis EssB homologue YukC (6Z0F) [43]. The C-terminus is missing from the structure and is represented by a blue line.

Fig. 2.

Model for the structural organisation of the T7SSb. A model for the assembled T7SSb, based on T7SSa architecture and known T7SSb intersubunit interactions. EssC FHA domains are required for association of EssA and EssB, but not EsaA. EsaA and EssC are thought to interact via their transmembrane domains and bacterial two hybrid studies suggest that EssA might recruit EsaB to the translocon [43].

Fig. 3.

T7SSb genetic organisation in different organisms. The genetic organisation of T7SSb loci is shown for selected species. For each species the overall organisation of the T7SS locus is shown at the top with conserved core components in blue/green and substrate and accessory clusters in red. The substrate and accessory clusters are highly variable within species, and in some species the 3′ end of essC varies with this downstream region. Genetic arrangements within the variable regions are shown for selected strains, with the essC variant for that strain given in brackets where applicable. The example arrangements shown are non-exhaustive, and diagrams are not to scale. Genes with frequently occurring conserved domains are colour coded. Some domains were not identified by Interpro but were classified by HHPred search. S. agalactiae T7SS loci can carry between zero and two copies of esxA, and esxA in S. gallolyticus (arrangement one) is on the opposite strand. * Denotes a pseudogene.

Fig. 4.

Structures of T7SS substrates. (a) Schematic showing the domain organisation of LXG toxins and WXG100 proteins. (b–d) AlphaFold models of T7SSb substrate complexes ( B. subtilis YxiD, S. intermedius TelC and S. aureus EsaD). In each model the LXG protein is coloured lilac, and the WXG100-like partners green, yellow and red. In (c) the EsaD sequence used for the prediction was truncated at 200 amino acids to remove the C-terminal domain which cannot be confidently predicted. The low-resolution volume of the EsaD-EsxB-EsxC-EsxD-EsaE-EsaG hexamer, determined by cryo-EM [98], is overlaid providing experimental support for the model. (e–f) Experimentally determined structures for two T7SSa substrates, in complex with their targeting chaperones ( M. tuberculosis PE5-PPE4-EspG3 [116], and M. tuberculosis EspB-EspK [119]). All structures are oriented with the export arm at the top.