The push-pull mechanism of bacteriophage O29 DNA injection

The mechanism of bacteriophage DNA injection is poorly understood, often considered a simple process, driven merely by the packing pressure inside the capsid. In contrast to the well-established DNA packaging mechanism of Bacillus subtilis phage O29, that involves a molecular motor formed by the connector and a viral ATPase, nothing is known about its DNA injection into the cell. We have studied this process measuring DNA binding of p6, a viral genome organization protein. The linear DNA penetrates with a right-left polarity, in a two-step process. In the first step similar to65% of the genome is pushed into the cell most probably by the pressure built inside the viral capsid. Thus, synthesis of viral proteins from the right early operon is allowed. This step is controlled, probably by bacterial protein(s) that slow down DNA entry. In the second step at least one of the viral early proteins, p17, participates in the molecular machinery that pulls the remaining DNA inside the cell. Both steps are energy-dependent, as treatment of cells with azide overrides the whole mechanism, leading to a deregulated, passive entry of DNA.