Federica Briani

Schitoviruses are widespread prokaryotic viruses that encapsidate a giant ( ~ 3500-residue) virion-associated RNA polymerase (vRNAP). During infection, vRNAP is expelled into Gram-negative bacteria, along with two additional ejection proteins, to assemble a transient DNA-ejectosome that becomes transcriptionally active, initiating viral replication. Here, we present an integrative structural analysis of the coliphage N4 vRNAP (gp50). We find that this 383 kDa enzyme is a multi-domain, single-chain RNA polymerase, structurally distinct from both compact single-chain RNAPs and large multi-subunit holoenzymes. vRNAP is composed of loosely connected domains and exhibits an intramolecular mode of allosteric regulation through its C-terminal domain. Comparative analysis of intact and genome-released virions identified gp51, which forms an outer-membrane complex, and gp52, which assembles a periplasmic tunnel. These proteins generate heterogeneous pores that facilitate the release of vRNAP. We further uncover a signaling hub in the phage tail, composed of the receptor-binding protein, tail tube, and tail plug, that detects receptor engagement and orchestrates the release of ejection proteins. We propose that the beads-on-a-string architecture of vRNAP enables the translocation of megadalton-scale protein complexes through the ~35 Å channel formed by the tail and ejection proteins. These findings establish N4 as a distinctive model for protein translocation through biological channels.

Pseudomonas aeruginosa bacteriophage (phage) DEV is a podovirus of the Schitoviridae family, related to the prototypical Escherichia coli phage N4. N4 uses the novel glycan receptor (NGR) surface glycan, presumably bound by the gp66 appendages, and the NGR transporter NfrA, recognized by the phage gp65 tail sheath, as receptors for adsorption. In contrast, DEV relies on the O-antigen moiety of lipopolysaccharide (LPS) as the primary receptor recognized by the gp53 long tail fibers. However, DEV can infect deep-rough strains that lack the O-antigen moiety by using another, still unknown receptor. Here, we provide evidence that the essential LPS transporter LptD serves as the DEV secondary receptor and that DEV gp54 is its cognate receptor-binding protein. gp54 is encoded within the essential gp56-gp55-gp54 operon, which also includes gp56 , the short tail fiber gene. Using cryogenic electron microscopy, AlphaFold modeling, and genetic analysis, we show that DEV gp56, gp55, and gp54 assemble into a receptor-binding fiber (RBF) positioned laterally to a previously uncharacterized tail plug protein, gp74. The DEV RBF is functionally equivalent to the N4 sheath protein gp65, which associates with the tail plug gp53. Thus, DEV and N4 both use a glycan and its surface-exposing transporter as receptors for adsorption. To our knowledge, this is the first example of a P. aeruginosa phage using an essential outer membrane protein as a receptor, with implications for phage therapy. IMPORTANCE: Pseudomonas aeruginosa phage DEV uses the O-antigen of lipopolysaccharide as its primary receptor. In this study, we found that LptD, an essential and highly conserved outer membrane protein, serves as the secondary receptor for DEV. This interaction is mediated by a specialized receptor-binding fiber composed of the DEV proteins gp54 , gp55 , and gp56 . We posit that the gp56-gp55-gp54 genes form a functional module, possibly disseminated via horizontal gene transfer among distantly related phages, involved in tail sealing and the regulated unplugging of the tail upon interaction with the bacterial receptor. Given the high conservation of receptor-binding proteins among phages in the DEV Litunavirus genus, we anticipate that other members of this genus may also use LptD as their receptor. Since Litunaviruses are actively explored for phage therapy, insights into the interaction between DEV and its receptors could help develop more effective and targeted phage-based treatments.

Schitoviruses are widespread prokaryotic viruses that encapsidate a giant (~3,500-residue) virion-associated RNA polymerase (vRNAP). During infection, vRNAP is expelled into Gram-negative bacteria, along with two additional ejection proteins, to assemble a transient DNA-ejectosome that becomes transcriptionally active, initiating viral replication. Here, we present an integrative structural analysis of the coliphage N4 vRNAP (gp50). We find that this 383 kDa enzyme is a multi-domain, single-chain RNA polymerase, structurally distinct from both compact single-chain RNAPs and large multi-subunit holoenzymes. vRNAP is composed of loosely connected domains and exhibits an intramolecular mode of allosteric regulation through its C-terminal domain. Comparative analysis of intact and genome-released virions identified gp51, which forms an outer-membrane complex, and gp52, which assembles a periplasmic tunnel. These proteins generate heterogeneous pores that facilitate the release of vRNAP. We further uncover a signaling hub in the phage tail, composed of the receptor-binding protein, tail tube, and tail plug, that detects receptor engagement and orchestrates the release of ejection proteins. We propose that the beads-on-a-string architecture of vRNAP enables the translocation of megadalton-scale protein complexes through the ~35 Å channel formed by the tail and ejection proteins. These findings establish N4 as a distinctive model for protein translocation through biological channels.

Human PNPase (hPNPase) is an essential RNA exonuclease located in mitochondria, where it contributes to RNA import from the cytoplasm, degradation of mitochondrial RNA and R-loop homeostasis. Biallelic mutations in the hPNPase PNPT1 gene cause different genetic diseases, ranging from hereditary hearing loss to Leigh syndrome. In this work, we used an Escherichia coli model to test the effects of four pathological PNPT1 mutations associated with diseases of different severity. Moreover, we generated a new human cell model by introducing PNPT1 mutations into 293T cells via CRISPR-Cas editing. Notably, the bacterial cells expressing the different mutant alleles exhibited similar phenotypes consistent with hPNPase loss of function. In contrast, the human cell model responded differently to the two mutations tested, with responses correlating with the severity of the respective pathologies. We interpreted the data derived from both models in the light of the in vitro RNA binding and degradation activity of the wild-type and mutated hPNPase variants. We found that all pathogenic mutations tested caused defects in protein assembly and affected the degradation and RNA binding efficiency to varying degrees. However, the severity of the conditions caused by different mutations did not correlate with the catalytic activity of the mutant proteins.

Human PNPase (hPNPase) is an essential RNA exonuclease located in mitochondria, where it contributes to RNA import from the cytoplasm, degradation of mitochondrial RNA, and R-loop homeostasis. Biallelic mutations in the hPNPase PNPT1 gene cause different genetic diseases, ranging from hereditary hearing loss to Leigh syndrome. In this work, we used an Escherichia coli model we recently developed to test the effects of four pathological PNPT1 mutations associated with diseases of different severity. Moreover, we generated a new human cell model by introducing PNPT1 mutations into 293T cells via CRISPR-Cas editing. Notably, the bacterial cells expressing the different mutant alleles exhibited similar phenotypes consistent with hPNPase loss of function. In contrast, the human cell model responded differently to the two mutations tested, with responses correlating with the severity of the respective pathologies. We interpreted the data derived from both models in the light of the in vitro RNA binding and degradation activity of the purified wild-type and mutated hPNPase variants. We found that all pathogenic mutations tested caused defects in protein assembly and affected the degradation and RNA binding efficiency to varying degrees. However, the severity of the conditions caused by different mutations did not correlate with the catalytic activity of the mutant proteins. Also, we demonstrated that human PNPase, like its bacterial orthologue, is capable of trimming ssDNA and some mutations inhibit such activity.

DEV is an obligatory lytic Pseudomonas phage of the N4-like genus, recently reclassified as Schitoviridae. The DEV genome encodes 91 ORFs, including a 3398 amino acid virion-associated RNA polymerase (vRNAP). Here, we describe the complete architecture of DEV, determined using a combination of cryo-electron microscopy localized reconstruction, biochemical methods, and genetic knockouts. We built de novo structures of all capsid factors and tail components involved in host attachment. We demonstrate that DEV long tail fibers are essential for infection of Pseudomonas aeruginosa but dispensable for infecting mutants with a truncated lipopolysaccharide devoid of the O-antigen. We determine that DEV vRNAP is part of a three-gene operon conserved in 191 Schitoviridae genomes. We propose these three proteins are ejected into the host to form a genome ejection motor spanning the cell envelope. We posit that the design principles of the DEV ejection apparatus are conserved in all Schitoviridae.

DEV is an obligatory lytic Pseudomonas phage of the N4-like genus, recently reclassified as Schitoviridae. The DEV genome encodes 91 ORFs, including a 3,398 amino acid virion-associated RNA polymerase. Here, we describe the complete architecture of DEV, determined using a combination of cryo-electron microscopy localized reconstruction, biochemical methods, and genetic knockouts. We built de novo structures of all capsid factors and tail components involved in host attachment. We demonstrate that DEV long tail fibers are essential for infection of Pseudomonas aeruginosa and dispensable for infecting mutants with a truncated lipopolysaccharide devoid of the O-antigen. We identified DEV ejection proteins and, unexpectedly, found that the giant DEV RNA polymerase, the hallmark of the Schitoviridae family, is an ejection protein. We propose that DEV ejection proteins form a genome ejection motor across the host cell envelope and that these structural principles are conserved in all Schitoviridae.

Phage therapy represents a promising strategy for curing bacterial infections refractory to antibiotics. However, the success rate of phage therapy may be lowered by the emergence of bacterial resistance to the phages used for therapy. In this work, we studied the resistance to the CK4 cocktail, which is a mixture composed of four phages able to cure Pseudomonas aeruginosa infections in animal models. CK4-resistant mutants were easily isolated from cultures grown in either a standard laboratory medium or an artificial sputum medium mimicking the composition of the airway fluid of cystic fibrosis (CF) patients, who are highly susceptible to P. aeruginosa chronic lung infections. In both cases, CK4-resistant mutants resulted in being defective in lipopolysaccharide (LPS) biosynthesis. Accordingly, all CK4 phages were unable to infect wzy mutants lacking the O-antigen polymerase. A survey of the other 15 P . aeruginosa phages isolated from different environmental sources showed that they all needed either wzy or the Type IV-pilus (T4P) biosynthetic gene pilQ for the infection. Overall, our data suggest that 16 out of the 19 analyzed Pseudomonas phages may use either the LPS or the T4P as a receptor. Interestingly, CK4-resistant mutants devoid of the O-antigen had strongly attenuated virulence in a zebrafish embryo infection model, and the lack of T4P also decreased virulence in zebrafish. With respect to isolates from patients with CF, phages not reproducing in the Δ wzy mutant had a wider host range than those requiring pilQ , suggesting that phages dependent on PAO1-type T4P may have limited therapeutic value for treating CF-related infections. IMPORTANCE: In this work, we identified the putative receptors of 16 Pseudomonas phages and evaluated how resistance to phages recognizing different bacterial receptors may affect the virulence. Our findings are relevant for the implementation of phage therapy of Pseudomonas aeruginosa infections, which are difficult to treat with antibiotics. Overall, our results highlight the need to modify natural phages to enlarge the repertoire of receptors exploited by therapeutic phages and suggest that phages using the PAO1-type T4P as receptor may have limited value for the therapy of the cystic fibrosis infection.