Over-Repair in Closterium: Increased Radioresistance Caused by an Earlier Exposure to Radiation

Abstract

The unicellular green fresh-water desmid Closterium moniliferum, when irradiated at high dose-rate with 10 MeV electrons, yields a survival curve with a threshold to a dose of about 10 krads, followed by a shoulder and an exponential decline. Previous exposure to electrons, x-rays, or 260 nm ultraviolet (UV) causes an increase in D_T and in the width of the shoulder by a factor of up to 2, but little or no change in slope. We call this over-repair and suggest that cells normally have an efficient mechanism for repair of sublethal damage and that this, or an additional mechanism, is stimulated by the first dose of radiation and then repairs subsequent radiation damage even more efficiently. This interpretation is consistent with the large effect of dose-rate on the value of D_T and the width of the shoulder in single-dose experiments.

The effectiveness of the first dose in stimulating over-repair was small but measurable after 30 rads of x-rays and increased to an optimum between 1 and 7 krads. It was not influenced by the rate at which the first dose was given. First doses of more than 10 krads did not stimulate over-repair. After a first dose of 2 krads, measurable over-repair was seen after an interval of 1 h and increased to a plateau at 4–6 h. Over-repair was slow or absent when cells were kept cold (0.5°C) between the stimulating first dose and the second or test dose, and was suppressed by cycloheximide. Over-repair stimulated by a first dose was still apparent even when 1 or 2 cell divisions (mitoses) occurred before the test dose was given.