Chromosomally-Encoded Hok/Sok Toxin/Antitoxin Systems Inhibit Phage Infection

The primary physiological role of toxin/antitoxin (TA) systems is phage defense; they are also used to stabilize mobile genetic elements. In the initial publication in 1996, plasmid-encoded Hok/Sok, a type I TA system (protein toxin, RNA antitoxin that inhibits toxin mRNA), was shown to inhibit T4 phage infection in Escherichia coli by reducing its efficiency of plating and burst size and by increasing the time for mature phage to form (latent period). This publication also identified that the mechanism of Hok/Sok inhibiting T4 was host transcription shutoff, arguably the most-general phage inhibition system since most phages impair host transcription. However, recently, the role of Hok/Sok in phage inhibition has been challenged. Therefore, here we investigated the role of four E. coli chromosomally-expressed, hok+/sok+ loci and the one orphan hok+ locus and found that the Hok proteins inhibit the infection of E. coli by BASEL phage 48, a Vequintavirus with myovirus morphology. Of the five Hok proteins, the greatest phage inhibition was by HokB, and infection by BASEL phages 34 and 39 were also inhibited by HokB. We also find BASEL phage 48 requires the WecB-generated receptor NGR for infection. Therefore, chromosomally-derived orphan Hok proteins, produced at physiological levels, are relevant for phage inhibition.