Optimization of near ultraviolet irradiation conditions for isolation of relaxed mutants of Escherichia coli.

We describe optimized conditions for isolation of relaxed mutants of bacteria with 4-thiouridine-containing tRNAs. The results presented here imply that besides the knowledge of the action spectra for near UV-induced growth inhibition of stringent and relaxed cells the fluence--fluence rate dependence of growth inhibition is an essential factor for optimizing the enrichment of relaxed mutants. We investigated systematically the dependence of growth inhibition of both stringent and relaxed strains of E. coli on the wavelength lambda, on the fluence F and on the fluence rate I within the ranges lambda = 301-365 nm, F = 0-48 kJ X m-2 and I = 0-60 W X m-2. The optimized conditions for selection of relaxed mutants of E. coli are lambda = 334 nm, F = 48 kJ X m-2 and I = 60 W X m-2. These optimized parameters determined for E. coli may be used in general to select relaxed mutants of other bacterial species by combining the turbidostat technique with near UV irradiation (Riesenberg et al. 1983).