Viral Ejection Proteins: Mosaically Conserved, Conformational Gymnasts

Bacterial Virus Forcing of Bacterial O-Antigen Shields: Lessons from Coliphages

In most Gram-negative bacteria, outer membrane (OM) lipopolysaccharide (LPS) molecules carry long polysaccharide chains known as the O antigens or O polysaccharides (OPS). The OPS structure varies highly from strain to strain, with more than 188 O serotypes described in E. coli. Although many bacteriophages recognize OPS as their primary receptors, these molecules can also screen OM proteins and other OM surface receptors from direct interaction with phage receptor-binding proteins (RBP). In this review, I analyze the body of evidence indicating that most of the E. coli OPS types robustly shield cells completely, preventing phage access to the OM surface. This shield not only blocks virulent phages but also restricts the acquisition of prophages. The available data suggest that OPS-mediated OM shielding is not merely one of many mechanisms of bacterial resistance to phages. Rather, it is an omnipresent factor significantly affecting the ecology, phage-host co-evolution and other related processes in E. coli and probably in many other species of Gram-negative bacteria. The phages, in turn, evolved multiple mechanisms to break through the OPS layer. These mechanisms rely on the phage RBPs recognizing the OPS or on using alternative receptors exposed above the OPS layer. The data allow one to forward the interpretation that, regardless of the type of receptors used, primary receptor recognition is always followed by the generation of a mechanical force driving the phage tail through the OPS layer. This force may be created by molecular motors of enzymatically active tail spikes or by virion structural re-arrangements at the moment of infection.

High-resolution cryo-EM structure of the Pseudomonas bacteriophage E217

E217 is a Pseudomonas phage used in an experimental cocktail to eradicate cystic fibrosis-associated Pseudomonas aeruginosa. Here, we describe the structure of the whole E217 virion before and after DNA ejection at 3.1 Å and 4.5 Å resolution, respectively, determined using cryogenic electron microscopy (cryo-EM). We identify and build de novo structures for 19 unique E217 gene products, resolve the tail genome-ejection machine in both extended and contracted states, and decipher the complete architecture of the baseplate formed by 66 polypeptide chains. We also determine that E217 recognizes the host O-antigen as a receptor, and we resolve the N-terminal portion of the O-antigen-binding tail fiber. We propose that E217 design principles presented in this paper are conserved across PB1-like Myoviridae phages of the Pbunavirus genus that encode a ~1.4 MDa baseplate, dramatically smaller than the coliphage T4.

Revolving hexameric ATPases as asymmetric motors to translocate double-stranded DNA genome along one strand

DsDNA translocation through nanoscale pores is generally accomplished by ATPase biomotors. The discovery of the revolving dsDNA translocation mechanism, as opposed to rotation, in bacteriophage phi29 elucidated how ATPase motors move dsDNA. Revolution-driven, hexameric dsDNA motors have been reported in herpesvirus, bacterial FtsK, Streptomyces TraB, and T7 phage. This review explores the common relationship between their structure and mechanisms. Commonalities include moving along the 5'→3' strand, inchworm sequential action leading to an asymmetrical structure, channel chirality, channel size, and 3-step channel gating for controlling motion direction. The revolving mechanism and contact with one of the dsDNA strands addresses the historic controversy of dsDNA packaging using nicked, gapped, hybrid, or chemically modified DNA. These controversies surrounding dsDNA packaging activity using modified materials can be answered by whether the modification was introduced into the 3'→5' or 5'→3' strand. Perspectives concerning solutions to the controversy of motor structure and stoichiometry are also discussed.

Diverse infective and lytic machineries identified in genome analysis of tailed coliphages against broad spectrum multidrug-resistant Escherichia coli

The emergence of multidrug-resistant (MDR) E. coli with deleterious consequences to the health of humans and animals has been attributed to the inappropriate use of antibiotics. Without effective antimicrobials, the success of modern medicine in treating infections would be at an increased risk. Bacteriophages could be used as an alternative to antibiotics for controlling the dissemination of MDR bacteria. However, before their use, the bacteriophages have to be assessed for the safety aspect. In this study, three broad host range highly virulent coliphage genomes were sequenced, characterized for infective and lytic potential, and checked for the presence of virulence and resistance genes. The genome sequencing indicated that coliphages ϕEC-S-21 and ϕEC-OE-11 belonged to Myoviridae, whereas coliphage ϕEC-S-24 belonged to the Autographiviridae family derived from the Podoviridae family. The genome size of the three coliphages ranged between 24 and 145 kb, with G + C content ranging between 37 and 51%. Coding sequences (CDS) ranged between 30 and 251 amino acids. The CDS were annotated and the proteins were categorized into different modules, viz., phage structural proteins, proteins associated with DNA replication, DNA modification, bacterial cell lysis, phage packaging, and uncharacterized proteins. The presence of tRNAs was detected only in coliphage ϕEC-OE-11. All three coliphages possessed diverse infective and lytic mechanisms, viz., lytic murein transglycosylase, peptidoglycan transglycosylase, n-acetylmuramoyl-l-alanine amidase, and putative lysozyme. Furthermore, the three coliphage genomes showed neither the presence of antibiotic resistance genes nor virulence genes, which makes them desirable candidates for use in phage therapy-based applications.

High-resolution cryo-EM structure of the Shigella virus Sf6 genome delivery tail machine

Sf6 is a bacterial virus that infects the human pathogen Shigella flexneri. Here, we describe the cryo-electron microscopy structure of the Sf6 tail machine before DNA ejection, which we determined at a 2.7-angstrom resolution. We built de novo structures of all tail components and resolved four symmetry-mismatched interfaces. Unexpectedly, we found that the tail exists in two conformations, rotated by ~6° with respect to the capsid. The two tail conformers are identical in structure but differ solely in how the portal and head-to-tail adaptor carboxyl termini bond with the capsid at the fivefold vertex, similar to a diamond held over a five-pronged ring in two nonidentical states. Thus, in the mature Sf6 tail, the portal structure does not morph locally to accommodate the symmetry mismatch but exists in two energetic minima rotated by a discrete angle. We propose that the design principles of the Sf6 tail are conserved across P22-like Podoviridae.