In vivo photoinactivation of Escherichia coli ribonucleotide reductase by near-ultraviolet light

NEAR-ULTRAVIOLET light (320–400 nm) is lethal to bacteria 1 , yeast cells 2 and mammalian cells 3 . A large body of evidence correlates cell death and survival with the induction and repair of pyrimidine dimers in the DNA of cells irradiated by far-ultraviolet light (200–300 nm) (refs 4–7). The induction of pyrimidine dimers has also been demonstrated by 365-nm near-ultraviolet light but at 7×10 5 -fold lower efficiency than by 254-nm far-ultraviolet light 8 . With the exception of certain Escherichia coli strains with multiple deficiencies in DNA repair 9,10 the small number of pyrimidine dimers produced by 365-nm near-ultraviolet light does not account for a significant fraction of the biological damage either in terms of lethality 11 or the inactivation of transforming DNA 12 . The killing of cells by near-ultraviolet light is oxygen dependent whereas that by far-ultraviolet light is oxygen independent 3,13 . The operational distinction between far- and near-ultraviolet light based on oxygen dependence of lethality coincides with the upper limit of ultraviolet light absorption by the DNA at about 310nm 14 and may reflect the involvement of a primary target site other than DNA in the killing of cells by near-ultraviolet light. In E. coli deoxyribonucleotides are formed de novo by the reduction of ribonucleoside diphosphates 15 . Two of the components of the ribonucleoside diphosphate reductase (RDP-reductase) complex, the non-haem iron protein subunit of the RDP-reductase 16 and the functionally linked flavoprotein thioredoxin reductase 17 have strong absorption in the near-ultraviolet region. I show here that near-ultraviolet irradiation of E. coli cells selectively destroys RDP-reductase activity in vivo and present evidence relating the loss of RDP-reductase to the loss of cellular viability and to the inability of irradiated cells to support the replication of DNA phages.