Bacteria are found in ongoing conflicts with rivals and predators, which lead to an evolutionary arms race and the development of innate and adaptive immune systems. Although diverse bacterial immunity mechanisms have been recently identified, many remain unknown, and their dissemination within bacterial populations is poorly understood. Here, we describe a widespread genetic element, defined by the Gamma-Mobile-Trio (GMT) proteins, that serves as a mobile bacterial weapons armory. We show that GMT islands have cargo comprising various combinations of secreted antibacterial toxins, anti-phage defense systems, and secreted anti-eukaryotic toxins. This finding led us to identify four new anti-phage defense systems encoded within GMT islands and reveal their active domains and mechanisms of action. We also find the phage protein that triggers the activation of one of these systems. Thus, we can identify novel toxins and defense systems by investigating proteins of unknown function encoded within GMT islands. Our findings imply that the concept of 'defense islands' may be broadened to include other types of bacterial innate immunity mechanisms, such as antibacterial and anti-eukaryotic toxins that appear to stockpile with anti-phage defense systems within GMT 'weapon islands'.

Bacteria use the type VI secretion system (T6SS) to deliver toxic effectors into bacterial or eukaryotic cells during interbacterial competition, host colonization, or when resisting predation. The identity of many effectors remains unknown. Here, we identify RIX, a new domain that defines a class of polymorphic T6SS cargo effectors. RIX, which is widespread in the Vibrionaceae family, is located at N-termini of proteins containing diverse antibacterial and anti-eukaryotic toxin domains. We demonstrate that RIX-containing proteins are delivered via T6SS into neighboring cells, and that RIX is necessary and sufficient for secretion. We show that RIX-containing proteins can also act as tethers, enabling the T6SS-mediated delivery of other cargo effectors by a previously undescribed mechanism. RIX-containing proteins significantly enlarge the repertoire of known T6SS effectors, especially those with anti-eukaryotic activities. Our findings also suggest that T6SSs may play a major, currently underappreciated, role in interactions between vibrios and eukaryotes.