Cultures of Escherichia coli will not grow in media containing very high specific activities of radiophosphorus P32, the inhibition of growth being due to the decay of assimilated P32 atoms. Experiments with a differentially labeled thymineless strain of E. coli show that the P32 disintegrations which occur in the bacterial deoxyribonucleic acid, i.e. in the nucleus, are mainly responsible for the inactivation of the cell. The kinetics with which radioactive bacterial populations are inactivated indicate that the function of several nuclei per bacterial cell must be eliminated by P32 decay before the ability to generate a colony is lost. The efficiency with which each P32 disintegration inactivates the nucleus in which it has occurred is calculated to be 0.02 (at –196°), i.e., similar in magnitude to the killing efficiency of P32 decay in bacteriophages.

P32 decay and thymine starvation cooperate in bringing about the death of individuals of the thymineless strain, from which observation it is inferred that "thymineless death" is likewise a nuclear inactivation.

The descendants of a non-radioactive bacterial culture grown for several generations in the presence of P32 and the descendants of a radioactive culture grown in the absence of P32 are inactivated by P32 decay in a manner which indicates that the phosphorus atoms of bacterial nuclei are dispersed among the progeny nuclei in their line of descendance.

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