Bacteriophage T7 DNA ejection into cells is initiated by an enzyme-like mechanism

In a normal infection about 850 bp of the bacteriophage T7 genome is ejected into the cell, the remainder of the genome is internalized through transcription by Escherichia coli and then T7 RNA polymerase. Rates of T7 DNA internalization by the E. coli enzyme in vivo are constant across the whole genome. As expected for an enzyme-catalysed reaction, rates vary with temperature and can be fitted to Arrhenius kinetics. Phage virions containing a mutant gp16, a protein known to be ejected from the phage capsid into the cell at the initiation of infection, allow complete entry of the T7 genome in the absence of transcription. The kinetics of DNA ejection from such a mutant virion into the bacterial cytoplasm have also been measured at different temperatures in vivo. Between 15 and 43 degrees C the entire 40 kb T7 genome is translocated into the cell at a constant rate that is characteristic for each temperature, and the temperature-dependence of DNA translocation rates can be fitted to Arrhenius kinetics. The data are consistent with the idea that transcription-independent DNA translocation from the T7 virion is also enzyme-catalysed. The proton motive force is necessary for this mode of DNA translocation, because collapsing the membrane potential while the T7 genome is entering the cell abruptly halts further DNA transfer.