Bacteriophages, or simply “phages,” are viruses that infect prokaryotes – bacteria and archaea. Phages come in many shapes and sizes, including filamentous Inoviridae such as phage fd pictured below
and icosahedral Microviridae such as phi-X 174 shown here:
Phages that infect hyperthermophiles seem to be even more bizarre, including bottle- and lemon-shaped particles. But your garden-variety bacteriophages also include the only classes of virus particles known to have “tails.” The tails are tube-like structures protruding from one of the capsid vertices, often with additional architecture like base plates, spikes, or thin fibers located at the tips. Phages with tails can be classified into three categories:
Podoviridae – short, non-contractile tails (above);
Myoviridae – sheathed, contractile tails(above);
or Siphoviridae – long, flexible, non-contractile tails (above).
These tails are non-motile, so they don’t function as a means of locomotion. The tail tips (and the more elaborate architecture found there) mediate the infection process upon contact with the appropriate surface receptor on the host cell, sort of like a key in a lock. For example, phage lambda, a Siphoviridae, uses the maltose receptor on E. coli as its gateway. Normally, E. coli uses the receptor to gather the sugar maltose from its suroundings, but lambda has co-opted this natural feature of its host cell surface biochemistry to its own purposes.
My question is this: why so many different kinds of tails? And why have a tail at all? The only phages for which tails seem necessary are the Myoviridae. Due to their contractile nature, the tail apparatus is essential in penetrating the cell membrane and in passing the viral nucleic acid into the host cell. But what about these other, non-contractile tail types? Rather than at the end of a tail, why not simply place the receptor recognition complex on one of the vertices – problem solved. Some of the Siphoviridae tails can be over 200 nm long. This seems like a liability, especially considering that if a phage particle loses its tail, it is rendered “dead” or inactivated. So, beyond the Myoviridae, what is the evolutionary advantage to a phage tail, and what is the advantage of the different types? There has to be one, or the tails wouldn't still be around. Are they essential in packaging the viral DNA? In chaperoning viral DNA into the host cell during infection? Are they useful in extending the “reach” of a given phage particle? I am still looking for reasonable answers. If you have any information on the subject, feel free to chime in!
2 comments:
So could the long sipho and myo tails effectively increase the 'reach' of the virus to contact the host cell. This would come in handy for environments with low host density. Of course there are exceptions, but Podos tend to be virulent and have large burst sizes. Siphos tend to be temperate and have smaller burst sizes. The smaller burst of siphos could mean smaller overall populations and reduce their host contact rate.
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