Radiation Resistance and Deoxyribonucleic Acid Base Composition of Micrococcus radiodurans

Abstract

Micrococcus radiodurans is a red-pigmented, non-sporing bacterium which is extremely resistant to both ionizing¹ and ultra-violet radiations². The mechanisms of resistance are not known. In 1962 Kaplan and Zavarine³ examined the relationship between the base composition of DNA in eight species of bacteria and their resistance to X-radiation. On plotting the X-ray dose which reduced the surviving bacteria to 10 per cent on the exponential part of the survival curve (the D10) against the guanine cytosine (GC) content, a linear relationship was obtained in which resistance and GC content were inversely related, that is, resistance increased with lowering of the GC content. This was shown to apply to bacteria the GC content of which varied between 34 and 67 per cent of the total bases. On the other hand, preliminary experiments by the same authors³ with ultra-violet radiation indicated the reverse situation, namely, a direct relation between the GC content and resistance to ultra-violet radiation, that is, resistance increased with increasing GC content. Similar results have been reported by Haynes⁴. It follows that extreme resistance to both ionizing and ultra-violet radiations should be incompatible in the same organism. Since M. radiodurans is very resistant to both types of radiation it cannot fulfil both of the criteria which have been postulated for characterizing radiation resistance.