Photoprotection of E. coli B/r: respiration, growth, macromolecular synthesis and repair of DNA.

— Photoprotecting effects of near UV radiations (300–400 nm, maximum at 360 nm) against far UV radiations (primarily 254 nm) have been studied in Escherichia coli B/r cells in minimal medium with glycerol as a carbon source. Near UV light (105 Jm-2) has a negligible effect on survival, but causes transitory inhibition of respiration, growth, DNA, RNA, and protein syntheses and cell division. Far UV (52 J m-2) reduces survival to about 0.5 per cent; respiration, growth and RNA and protein syntheses proceed for about 60 min, after which they nearly cease for several hours. Near UV given before this fluence of far UV increases survival 10-fold and the above processes resume at times and with kinetics characteristic of those produced by lower fluences of far UV. Single-strand breaks appear in the DNA of both unprotected and photoprotected cells; repair of the breaks is essentially complete in protected but not unprotected cells. The viability kinetics for far-UV-irradiated cells with and without photoprotecting treatment are identical except that the curve for the latter is displaced upward about 1 log; exponential increases (cell division) begin at 120 min in each case. The data suggest that, in B/r cells grown under our particular conditions, namely in minimal medium with glycerol, photoprotection is not the result of growth or division delays, but reflects an increased repair capability due to continued respiration.