The picture of this RNA polymerase and DNA molecule has enabled the team to investigate how the RNA polymerase initiates transcription of phage DNA from inside a bacterial cell. "When a phage injects its DNA into a bacterial cell, the amount of its DNA is miniscule compared to the amount of host DNA," said Murakami. "We wanted to find out what prevents the N4 RNA polymerase from binding to the bacterial host's DNA rather than to the phage's DNA."
It turns out that the N4 RNA polymerase is able to respond only to DNA that is shaped like a hairpin. Part of the N4 phage's DNA is shaped like a hairpin, whereas the E. coli bacterium's DNA is not shaped like a hairpin. Once the N4 RNA polymerase interacts with the phage's hairpin DNA, it begins to change its shape from a fisted form to a cupped form. By opening up, the RNA polymerase exposes its active site, which allows it to begin the transcription process.
While the researchers determined that the N4 RNA polymerase must change its form in order to bind to the phage DNA, they also found that this transformation isn't the polymerase's first as it progresses through the steps of phage infection. The team found that the polymerase must change form in order to squeeze through the phage's tiny injection tube as it is injected into the E. coli cell. "The diameter of the tube is narrower than the diameter of RNA polymerase," said Murakami. "This means that the enzyme must be unfolded into a longer and thinner structure in order to fit through the tube
|Contact: Barbara K. Kennedy|