The Use of Auxotrophic Mutants to Study Restoration in Escherichia Coli B after Ultra-violet-irradiation

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Abstract
To elcidate the mechanism by which cells of E. coli B may be rescued after ultra-violet-irradiation, mutants with specific nutritional requirements were used, so that certain synthetic pathways could be blocked at will. If mutants requiring an amino acid are incubated for a period after ultra-violet irradiation on a medium lacking that amino acid, survival is greatly increased above the level observed when the required amino acid is supplied immediately. Little rescue is observed if the cells are incubated on complete medium for 30 min before the amino acid is withdrawn. There is no rescue of cells which require thymine when thymine is withheld during the initial post-irradiation period. Considerable rescue is observed if all nutrients are withheld for a period after irradiation. The results support the postulate that ultra-violet damages at least two sites in the cell. Rapid amino-acid metabolism brings to light damage to one of these sites.