GMT systems define a new class of mobile elements rich in bacterial defensive and offensive tools

Conflicts between bacteria and their rivals led to an evolutionary arms race and the development of bacterial immune systems. Although diverse immunity mechanisms were recently identified, many remain unknown, and their dissemination within bacteria is poorly understood. Here, we describe a widespread genetic element, defined by the presence of the Gamma-Mobile-Trio (GMT) proteins, that serves as a bacterial survival kit. We show that GMT-containing genomic islands are active mobile elements with cargo comprising various antibacterial type VI secretion system (T6SS) effectors and anti-phage defense systems. We identify four new anti-phage defense systems encoded within GMT islands. A thorough investigation of one system reveals that it is triggered by a phage capsid protein to induce cell dormancy. Our findings underscore the need to broaden the concept of ‘defense islands’ to include also offensive tools, such as T6SS effectors, which we contend should be regarded as part of the bacterial immune system. ### Competing Interest Statement The authors have declared no competing interest.