Action Spectra for the Ultraviolet and Visible Light Inactivation of Phage T7: Effect of Host-Cell Reactivation

Kinetics of the inactivation of phage T7 by six ultraviolet and visible light wavelengths of the far (below 320 nm) and the near uv (above 320 nm) were studied, with and without host-cell reactivation. Inactivations were always exponential with the three shorter wavelengths (254,313, and 334 nm), whereas with the longer wavelengths (365, 405, and 460 nm), a small shoulder (extrapolation number <2) was consistently obtained. The host-cell reactivation sector was prominent with 254 and 313 nm of radiation, reduced with 334 nm, and either trivial or absent with the three longer wavelengths. Action spectra for the inactivation revealed small shoulders in the near-uv region, both with and without host-cell reactivation. A comparison of single-strand break (alkali-labile bond) induction by 365 nm of radiation in phage T4, compared with lethality in phage T7, revealed that a frequency of 0.3 single-strand breaks may occur per lethal hit.