Viruses come in all shapes and sizes. From the very small, such as the picornaviruses or the parvoviruses, to the very large like mimivirus, or the herpesviruses, and poxviruses. These large viruses are not just large in physical size, but in the size of their genomes as well.At the recent 2008 ASM General Meeting, Roger Hendrix of the University of Pittsburgh, laid forth a rather interesting hypothesis as to how large genomes, and the capsids that hold them came into existance and how they managed to be competitive in the gene pool.
Using phage P1 as an example, we know that larger capsids can be created "simply" by a single mutation allowing capsid subunits (capsomers) to come together in a quasi-equivilant matrix that is larger than the previous. This matrix follows the mathematical model set by Casper and Klug, and has discrete sizes (triangulation numbers, such as T=1,3,4,7,13). An increase in T number, as in our P1 example, causes a dramatic increase in capsid volume. Hendrix proposes that a mutation causing a such a shift acts as an evolutionary ratchet, and therefore smaller capsid sizers would no longer be available.
Now that we have a larger capsid, the phage now has the ability to package much more DNA. Not only does it have the ability, but in many cases, the phage MUST package DNA until its capsid is filled (headfull-packaging). With a larger capsid, phages who package via headful mechanisms now must package more DNA creating a greater amount of redundancy in its genome.
Hendrix explained that a greater amount of terminal redundancy leads to greater resistance from DNA damaging agents, specifically UV light. Although some in the session contended this, Hendrix described large amounts of genomic redundancy as an evolutionary advantageous trait for phages which live on the surface of the ocean and soil.
Furthermore, the extra space in the genome acts as a virtual genetic laboratory to aquire and mutate genes without disrupting the ability of the phage to survive. Gene aquisitions and subsequent mutations could create genes which provide some sort of marginal (or large) benefit to the phage or the host it infects.
With a simple click of the ratchet and a headfull of DNA, the role that large phages play in novel gene development are only now beginning to become clear.
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