Torsional tension in intracellular bacteriophage T4 DNA: Evidence that a linear DNA duplex can be supercoiled in vivo

Torsional tension in intracellular bacteriophage T4 DNA and host cell DNA was measured in infected Escherichia coli cells using the trimethylpsoralen photobinding assay. Early in infection superhelical tension in the host E. coli DNA was gradually reduced until at 8 min post-infection there was no detectable tension. Negative torsional tension in the T4 DNA appeared transiently, reaching a maximum 4 to 6 min post-infection (at 32 °C) and declined to undetectable levels by 10 min. The maximum level of tension averaged over all infecting T4 DNA molecules was equivalent to superhelical density of about σ = −0.03.