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 host's 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.
The researchers used fluorescently labeled phage DNA to investigate in real time (and with a resolution time of 750 milliseconds) the dynamics of DNA ejection from single phages. The ejected DNA was measured at different stages of the ejection process after being stretched by applied hydrodynamic flow. The study demonstrated that DNA release is not an all-or-none process, but rather is unexpectedly complex. DNA release occurred at a very high rate, reaching 75,000 base pairs of DNA/sec, but in a stepwise fashion. Pausing times were observed during ejection, and ejection was transiently arrested at definite positions of the genome in close proximity to genetically defined physical interruptions in the DNA. The authors discuss the relevance of this stepwise ejection to the transfer of phage DNA in vivo.