Rhs toxins

Rhs toxins belong to the polymorphic toxin category of bacterial exotoxins.^[1] Rhs proteins are widespread and can be produced by both Gram-negative and Gram-positive bacteria.^[2] Rhs toxins are very large proteins of usually more than 1,500 aminoacids with variable C-terminal toxic domains. Their toxic activity can either target eukaryotes or other bacteria.

Domain architecture

In their large N-terminal region, Rhs toxins comprise RHS/YD repeats in various number (PF05593) (RHS meaning Rearrangement Hot Spot) ^[3] and another "RHS-repeats associated core" domain (PF03527). In contrast, their C-terminal regions are shorter and harbor highly variable C-terminal domains including many domains with a predicted nuclease activity.^[2]

Function

Anti-eukaryotic activity

These toxins encompass Rhs toxins of insect pathogens with an activity against insects.^[4] This group also include Rhs toxins with an activity against human phagocytic cells that contribute to pathogenesis of Pseudomonas aeruginosa.^[5]

Anti-bacterial activity

A role in inter-bacterial competition has been demonstrated for the plant pathogen Dickeya dadantii and for the human pathogen Escherichia coli.^[6][7]

When a polymorphic toxin with anti-bacterial activity is produced by a bacterial strain, this strain is protected by a specific immunity protein encoded by a gene immediately downstream of the toxin gene.^[1]

Delivery

Some Rhs toxins such as the previously mentioned system in Dickeya dadantii appear to be dependent on the type VI secretion system for delivery into neighbouring cells.^[6] PAAR domain toxins such as Rhs appear to form the sharp tip of the type VI secretion system being attached to the VgrG of the secretion apparatus.^[8] The C-terminal toxins of Rhs may vary to diversify the antimicrobial activity of the type VI secretion system.^[7]