Evidence for two geometrically distinct targets mediating arginine reversion in E. coli

Calculated energy deposits from the decay of tritium and other radioisotopes incorporated into bacteria

Transmutation of the radioisotope tritium occurs with the production of a low energy electron, having a range in biological material similar to the dimensions of a bacterium. A computer program was written to determine the radiation dose distributions which may be expected within a bacterium as a result of tritium decay, when the isotope has been incorporated into specific regions of the bacterium. A nonspherical model bacterium was used, represented by a cylinder with hemispherical ends. The energy distributions resulting from a wide variety of simulated labeled regions were determined; the results suggested that the nuclear region of a bacterium receives on the average significantly different per decay doses, if the labeled regions were those conceivably produced by the incorporation of thymidine-(3)H, uracil-(3)H, or (3)H-amino acids. Energy distributions in the model bacterium were also calculated for the decay of incorporated (14)carbon, (35)sulfur, and (32)phosphorous.

Nonsense suppression in a multiauxotrophic derivative of Escherichia coli 15T-: identification and consequences of an amber triplet in the deoxyribomutase gene

Previously, arginine revertants of Escherichia coli WWU, a derivative of E. coli 15T(-), have been subdivided by two independent methods: (i) the streak morphology on nutrient agar, and (ii) the pattern of phage growth using amber and ochre mutants of bacteriophage T4. In the first assay, revertants were subdivided into two classes according to the appearance of streaks after incubation on nutrient agar, a thick, even line of growth defining normal revertants and a thin, irregular line defining aberrant revertants. In the second assay, revertants were classified by the suppressors they contained. The present work demonstrates that revertants containing an amber suppressor show the aberrant morphology and are also able to catabolize thymidine for energy and carbon. This is in contrast to the parent WWU containing no suppressor, which shows a normal morphology and cannot utilize thymidine as an energy source. Revertants containing no suppressor, isolated specifically for their ability to catabolize thymidine, show an aberrant morphology. Together, these results indicate that the aberrant morphology results from suppression of an amber triplet in a gene of the thymidine catabolic pathway. Enzyme assays show the amber triplet to be in the gene specifying deoxyribomutase. It is suggested that the aberrant arginine revertants are analogous to high thymine-requiring mutants and that, in general, high and low thymine-requiring mutants differ from one another in their ability to catabolize deoxyribose-1-phosphate.