Studying phage, a primitive class of virus that infects bacteria by injecting its genomic DNA into host cells, researchers have gained insight into the driving force behind this poorly understood injection process, which has been proposed in the past to occur through the release of pressure accumulated within the viral particle itself.
Almost all phages (also known as bacteriophages) are formed of a capsid structure, or head, in which the viral genome is packaged during morphogenesis, and a tail structure that ensures the attachment of the phage to the host bacteria. A common feature of phages is that during infection, only their genome is transferred to the bacterial hosts cytoplasm, whereas the capsid and tail remain bound to the cell surface. This situation is very different from that found in most eukaryotic viruses, including those that infect humans, in that the envelope of these viruses fuses with the host plasma membrane so that the genome is delivered without directly contacting the membrane.
Phage nucleic acid transport poses a fascinating biophysical problem: Transport is unidirectional and linear; it concerns a unique molecule the size of which may represent 50 times that of the bacterium. The driving force for DNA transport is still poorly defined. It was hypothesized that the internal pressure built during packaging of the DNA in the phage capsid was responsible for DNA ejection. This pressure results from the condensation of the DNA during morphogenesis – for example, another group recently showed that the pressure at the final stage of encapsulation for a particular bacteriophage reached a value of 60 atomospheres, which is close to ten times the pressure inside a bottle of champagne. In the new work reported this week, researchers have evaluated whether the energy thus stored is sufficient to permit phage DNA ejection, or only to initiate that process.
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