CHANGES IN U.V. SURVIVAL CURVES OF ESCHERICHIA COLl B/r CONCOMITANT WITH CHANGES IN GROWTH CONDITIONS

For E. coli B/r u.v.‐irradiated while in logarithmic growth, the nature of the dose‐response curve was strongly dependent on both pre‐ and post‐irradiation conditions of growth. Survival curves for cells grown in nutrient medium, or minimal medium with glucose, and plated immediately after irradiation, demonstrated an initial insensitive or ‘shoulder’ region provided the plating medium was such that no derepression was required of operons controlling inducible enzyme systems. If, however, such derepression was called for, survival curves were of exponential form. Delay in plating resulted in the return of the survival curve to the shouldered form even when ‘shift‐down’ media were used. Of those cells grown before u.v.‐irradiation in minimal media and plated thereafter with the same sugar as carbon source, only those grown with glucose (or lactose) demonstrated the shouldered survival curve.