Identifying a type of toxic effectors exported by Streptococcus suis to enhance competitive fitness

<i>Streptococcus suis</i> poses a significant threat to both pig farming and public health, causing severe disease such as septicemia, meningitis and streptococcal toxic shock-like syndrome. Type VII secretion system (T7SS) delivers toxic effectors to play a crucial role in interbacterial competition and is vital for the zoonotic pathogen <i>S. suis</i> to colonize host tonsils effectively. Here we identified a type of hypothetical T7SS effectors in <i>S. suis</i>, seeming to be the fragmented toxins cut off N-terminal YeeF domain, redesignated as MSE-ExTs. MSE (marker for searching effectors) is a conserved sequence at the N-termini of modular effectors showing a diverse range of toxicities targeting NAD⁺. EssC is a core component of T7SS apparatus, while genetic rearrangements frequently occur within T7SS loci in <i>S</i><i>.</i><i> suis</i>, leading to the duplication and truncation of EssC. Most MSE-ExTs (MSE fusing exported toxins) are located downstream of a truncated EssC and lack cognate WXG100-like proteins, but still exported to mediate interbacterial antagonism for gaining a competitive advantage over the bacterial isolates from tonsil microbiota. Neither deletion of truncated<i> essC2</i> nor <i>essC3</i> could not diminish the MSE-ExT1 delivery, while the full-length EssC1 encoded in T7SS core locus was required for the lethality of MSE-ExT1 to sensitive bacterial cells. MapC2, an upstream small helical protein, shares a nearly identical C-terminal 50 aa sequence with EIC-CR (C-terminal conserved region of effector-paired immunity protein). This conserved fragment harbours a “YxxxD” targeting signal and interact with the D1 ATPase domain of the non-neighboring EssC, thereby activating the secretion of MSE-ExTs. These findings suggest that MapC2 and EIC-CR mediate the recruitment of effector-immunity pairs to T7SS apparatus under the absence of a full-length cognate EssC and WXG100-like proteins. This alternative strategy facilitates effectors’ delivery, even for fragmented substrates resulting from genetic rearrangements, highlighting its importance in ensuring the functionality of T7SS.