Escherichia coli uses several defense systems against T4 phage parasites, one of which is shown here in a colored electron micrograph image.
IMAGE: DEPARTMENT OF MICROBIOLOGY, BIOZENTRUM, UNIVERSITY OF BASEL/SCIENCE SOURCE
Bacteria have diverse mechanisms to protect themselves against phage. Some, such as the CRISPR-Cas system, are always ready to recognize and eliminate invaders. Others, such as toxin-antitoxin systems, are only activated after phage infection. Guegler and Laub investigated the toxIN system that protects Escherichia coli against several bacteriophages. The toxin, toxN, is a ribonuclease and the antitoxin, toxI, is an RNA with an array of repeats. Under normal conditions, toxN cuts toxI and binds the single motif. Infection by the phage T4 shuts off host transcription, including transcription of toxIN. Because toxI is unstable, toxN is released to cleave mRNA in the cell, which by this time is mainly phage derived. This prevents the production of new phage particles. Thus, by appropriating the host’s replicative machinery, phage also risk releasing their toxIN nemesis.
Mol. Cell 10.1016/j.molcel.2021.03.027 (2021).