Bacterial viruses, or bacteriophages, are ubiquitous organisms spanning very different ecological niches. Although genome comparison fails to show extensive relationship among bacteriophages, recent structural studies reveal a high degree of similarities. Most bacteriophages present an icosahedral proteinaceous head, which contains the nucleic acid, either
deoxyribonucleic acid (DNA)
or
ribonucleic acid (RNA)
. Exceptions to this rule are few cases where a lipid envelope forms part of the head, and those other cases where the head presents a filamentous geometry. The way bacteriophages infect the host cell is the basis of a main difference among them: one group (Caudovirales) has a specialised structure (the tail) that is responsible for the recognition of the host cell and the viral genome delivery, whereas those bacteriophages without tail present a variety of infecting strategies. In this study we will deal with the main common characteristics of bacteriophage heads and tails supporting their common evolutionary origin.
Key Concepts:
Bacteriophages share extended structural and functional similarities.
Bacteriophages are excellent examples of optimisation of genetic information to carry out complex functions.
Most bacteriophages enclose their nucleic acid in a protein container (built by multiple copies of one (or a few) proteins), and in a few cases they may include lipid envelopes.
The most conserved geometry in bacteriophage heads is icosahedral.
Double stranded DNA icosahedral bacteriophages follow a common assembly pathway.
The Caudovirales group (bacteriophages with icosahedral heads, dsDNA and a tail) are the most abundant virus type.
The packaging of dsDNA inside Caudovirales requires energy conversion (ATP hydrolysis) into mechanical translocation, and it is carried out by a dedicated viral machinery.
The maturation of icosahedral proheads into viral heads involves drastic reorganisations of the capsid components leading to more stable particles.
The tail of Caudovirales is a sophisticated machinery involved in cell recognition and nucleic acids delivery.
There are three main architectural designs of bacteriophage tails, each one adapting to different host interaction strategies.