T7 Phage Uses a Hyperpromiscuous Kinase to Break Through Bacterial Immunity

Bacteriophages and bacteria are locked in one of biology’s oldest evolutionary conflicts. Bacteria continually acquire new immune systems capable of detecting, blocking or destroying invading viruses, while phages evolve countermeasures that allow them to regain access to their hosts. A new study published in Nature has now revealed an unusually aggressive strategy used by bacteriophage T7: rather than precisely disabling one bacterial defence at a time, the virus releases a kinase capable of modifying an extraordinary fraction of the proteins inside the infected cell.

Illustration depicting how T7 kinase, a phage enzyme, modifies many proteins inside an infected bacterium, helping shut down its defence mechanisms. Credit: Daniela Velasco/EMBL

The work, led by researchers from the European Molecular Biology Laboratory in Heidelberg and collaborators, identifies the T7 protein kinase T7K, also known as gp0.7, as a remarkably broad anti-defence weapon. T7K has been known since the 1970s and was previously thought to phosphorylate a comparatively small number of host proteins involved in processes such as transcription, translation and RNA metabolism. Modern phosphoproteomics reveals a dramatically different picture. T7K behaves as a hyperpromiscuous kinase capable of phosphorylating proteins across almost the entire bacterial proteome without the sequence-motif specificity normally associated with kinase signalling.

During wild-type T7 infection, the researchers detected 19,532 phosphopeptides, compared with only a few hundred in uninfected bacteria or cells infected with T7 mutants lacking a functional kinase. The modification affected serine, threonine and tyrosine residues and showed essentially no sequence preference. Within five minutes of infection, more than 15,000 phosphopeptides could be mapped to 2,093 bacterial proteins and 33 phage proteins. Around 70% of the expressed host and phage proteomes were targeted in a T7K-dependent manner.

This is particularly striking because T7 carries only a single kinase of this type. The researchers found that its proteome-wide phosphorylation density exceeded that seen in datasets representing the collective activity of the hundreds of kinases encoded by human cells. In molecular terms, the virus appears to sacrifice the exquisite target specificity usually associated with cellular signalling in favour of speed and overwhelming biochemical activity.

Yet T7K is not simply modifying proteins at random. The study uncovered a second layer of organization that explains how such apparently indiscriminate activity can nevertheless benefit the phage.

The C-terminal region of T7K, known as the shutoff or SO domain, was found to interact with DNA. This effectively brings the kinase into proximity with proteins associated with nucleic acids. As a result, although T7K can phosphorylate a huge variety of substrates, DNA- and other nucleic-acid-binding proteins are disproportionately modified at high stoichiometry — meaning that a large fraction, and in some cases nearly the entire cellular population, of a particular protein can become phosphorylated. The SO domain therefore acts as a molecular targeting mechanism for an otherwise extremely promiscuous enzyme. Removing this domain strongly reduced proteome-wide phosphorylation and altered the kinetics of T7K activity.

That preference has profound consequences because many bacterial antiphage systems depend directly on DNA, RNA or proteins bound to nucleic acids. Bacterial immunity has expanded far beyond the classical restriction-modification and CRISPR-Cas systems; hundreds of defence-system families are now known. Most previously characterized phage anti-defence proteins are relatively specialized, targeting one defence mechanism or closely related pathways. T7K appears to represent something different: a broadly acting strategy capable of disrupting several unrelated forms of bacterial immunity.

The researchers demonstrated the mechanism experimentally using Retron-Eco9 and DarTG1. Retron systems can trigger abortive infection, sacrificing the infected bacterium before the phage completes replication. During T7 infection, the Retron-Eco9 toxin RcaT accumulated 17 confidently identified phosphorylation sites. Several were located close to residues important for its activity. Remarkably, phosphomimetic mutations designed to reproduce the effect of phosphorylation at individual sites were sufficient to abolish Retron-Eco9-mediated defence.

T7K also interfered with DarTG1, a toxin-antitoxin defence system in which DarT modifies invading phage DNA and prevents replication. Both components became extensively phosphorylated. These experiments support a model in which the kinase does not need to recognize the biochemical mechanism of every individual defence system. Instead, it exploits a vulnerability shared by many of them: their dependence on nucleic-acid-associated proteins.

One of the most important aspects of the study was the decision to move beyond standard laboratory E. coli. Laboratory strains have often been selected for genetic tractability and tend to possess relatively limited antiphage defence repertoires. This had probably helped conceal the true importance of T7K for decades.

The researchers therefore screened 513 natural E. coli isolates, including laboratory, commensal and pathogenic strains. Fifty-four could be infected by T7 under the experimental conditions, 22 showed evidence of defence, and 17 produced sufficiently reproducible phenotypes for further analysis. Among those 17 strains, removing T7K weakened T7’s ability to overcome bacterial defence in six — approximately 35% — while ten showed little effect and one actually defended more effectively when the kinase was present. In two genetically distinct strains, the contribution of T7K to overcoming defence exceeded three orders of magnitude.

The result is important precisely because the six affected isolates did not share one obvious defence system that could explain the phenotype. The authors therefore argue that T7K probably interferes with a wider and still incompletely identified collection of bacterial immune mechanisms. At the same time, the exceptional strain that performed better in the presence of T7K is a warning against treating the kinase as a universal anti-immunity switch: bacterial defence systems may evolve resistance to phosphorylation or could potentially exploit the kinase’s activity as an infection signal.

The phenomenon also extends beyond T7. When the researchers examined other Autographiviridae phages carrying T7K homologues, several produced similarly extensive phosphorylation during infection. Database searches identified numerous related kinases, predominantly in phages, and the closest homologues retained the same unusual architecture. However, an unrelated kinase carried by phage P1 did not show comparable proteome-wide promiscuity, meaning this is not a general property of every phage kinase.

At the structural level, T7K may achieve its extraordinary activity through another unusual feature. Most conventional protein kinases contain a conserved DFG motif involved in switching the catalytic centre between active and inactive conformations. T7K instead carries a DPV motif at the equivalent position. Structural modelling suggests that this configuration may favour an active-like arrangement of the catalytic site and contribute to the enzyme’s extreme processivity. T7 eventually restrains this potentially destructive activity through autophosphorylation, shutting the kinase down within roughly six minutes after infection.

This temporary nature is probably essential. T7 still depends on bacterial machinery to manufacture viral proteins and assemble progeny phages. Permanently destroying cellular function immediately after entry would therefore be counterproductive. Instead, the virus appears to unleash an intense but short-lived wave of phosphorylation early in infection, disrupting bacterial defensive machinery before switching the kinase off and allowing the remainder of the infection programme to proceed. The authors describe this as a new paradigm for how phosphorylation can be used to disable molecular systems rather than regulate them conventionally.

For phage therapy, the findings are especially interesting because bacterial immunity is increasingly recognized as an important determinant of host range. A therapeutic phage may appear effective against one isolate of a pathogen yet fail against another carrying a different combination of defence systems. Pathogenic bacterial populations are extraordinarily diverse in this respect, meaning that adsorption receptors are only one component of phage susceptibility.

Broad anti-defence mechanisms such as T7K could therefore eventually become valuable elements in phage engineering. Rather than designing a different countermeasure against every individual bacterial immune system, one possibility would be to equip therapeutic phages with broadly acting modules capable of neutralizing several defence pathways simultaneously. EMBL researchers specifically suggest that such strategies could help phages remain effective against genetically diverse strains of the same pathogen.

That therapeutic interpretation nevertheless remains prospective. This study did not test an engineered T7K-containing therapeutic phage in patients, nor demonstrate improved treatment outcomes in an animal infection model. The work establishes a molecular anti-defence mechanism and shows that it can substantially influence infectivity across natural bacterial isolates. Turning that biological insight into a safe and predictable engineering strategy will require determining how transferable the kinase is between phages, whether hyperphosphorylation creates fitness costs or unintended effects, and which bacterial defence architectures remain resistant to it.

What the study does establish is that the molecular arsenal carried by phages may contain far broader weapons than previously recognized. T7K does not merely evade one immune pathway. For a brief period after infection, it dramatically rewrites the phosphorylation state of the bacterial cell and preferentially disables proteins located where many bacterial defence systems operate.

After almost five decades of studying T7 kinase, modern proteomics has therefore revealed a function that conventional laboratory systems largely concealed. And for phage therapy, the discovery reinforces an increasingly important idea: understanding and manipulating phage anti-defence systems may be just as important as identifying the bacterial receptors that determine whether a phage can infect its target in the first place.

Sources :

EMBL — A viral ‘loose cannon’ enzyme helps phages shut down bacterial defences

Nature — Pervasive phosphorylation by phage T7 kinase disarms bacterial defences

The Nature article was published on 19 August 2026 and is open access. Its authors are Tara Bartolec, Karin Mitosch, Clément Potel and colleagues, with Athanasios Typas and Mikhail M. Savitski as corresponding authors. https://www.nature.com/articles/s41586-026-10934-5