Structure and Function of Bacteriophages
Bacteriophages, or phages, are viruses with an exquisitely evolved structure to accomplish their goals. These goals are recognizing a suitable host bacterium, profiting from the host metabolism, and producing multiple progeny phages that are stable enough to survive until they find a new host bacterium to infect. Their genomes consist of single-stranded RNA, double-stranded RNA, single-stranded DNA, or double-stranded DNA, depending on phage type. They store their genome in highly symmetric protein capsids to protect it from degradation. Often these capsids are icosahedral, but helical and other shapes are also used. Tectiviridae and Corticoviridae have an internal lipid membrane, while Cystoviridae sport an outer membrane layer. Phages with tails, belonging to the Caudovirales order, are the most commonly encountered bacteriophages and have icosahedral or prolate capsids. In addition to the capsid, phages need a host cell recognition apparatus. The small icosahedral Leviviridae have a single minor capsid protein for this purpose. More complex phages dedicate multiple proteins to host cell recognition, and examples of this are the helical Inoviridae and the icosahedral Tectiviridae, Corticoviridae, and Cystoviridae. The Caudovirales have highly efficient tail protein complexes for DNA transfer. These tails are flexible (Siphoviridae), extensible (Podoviridae), or contractile (Myoviridae). Apart from elements designed for genome protection, host recognition, and genome transfer, more complicated phage particles may contain proteins for environmental sensing, binding to suitable matrices where host bacteria are likely to be encountered, and other functions.
Structure figures were generated using the PYMOL Molecular Graphics System (Schrödinger LLC) and UCSF CHIMERA (Pettersen et al. 2004). The research in our lab is funded by grants BFU2017-82207-P from the Spanish Ministry of Science, Innovation and Universities, State Agency of Research, co-financed by the European Regional Development Fund of the European Union. We thank Antonio Pichel for help with the sections on the Microviridae and Cystoviridae and for preparing Figs. 5 and 9. We are also grateful to Carmen San Martín (CNB-CSIC) and Carmela García-Doval (University of Zurich) for proofreading, to Don Marvin for advice on inovirus structure, and to Petr Leiman (University of Texas Medical Branch) for advice on figures; any remaining mistakes are the responsibility of the authors.
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