A genetical study of thymineless mutants ofE. coliK12

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Abstract
The 150 independently isolatedthy−mutants ofE. coliK12 Hfr 3.OSO were studied genetically and phenotypically. Variants were found among the mutants in respect to the lag period of thymineless death, and temperature sensitivity. The latter correlates with mutations located at a specific site on the genetical map.Thethylocus is located between thecysandser/glygenes, and is a linear structure where 134thymutants are distributed over more than 17 sites. The site distribution of the mutants is not regular: about a half of them (62) are localized within one site and all these are temperature-sensitive.Two further genes involving utilization of thymine—tlrandtd—were found. Mutations oftlrlead to a reduced thymine requirement (0·5 μg./ml.instead of 20 μg./ml.). A mutation oftdresults in thymidine sensitivity.This latter character is expressed when thetd-sallele is transferred intoE. coliK12, prototrophic for thymine, by conjugation. Thymidine inhibition can be reversed by the addition of any riboside to the growth medium. Both genes map at the proximal end of the Hfr 3.OSO chromosome and are linked with thethrgene. The most probable gene order is:tlr-td-thr.The following results have been obtained from14C-thymine incorporation experiments with wild-type cells, as well as withthy−tlr+andthy−tlr−cells: (1) Wild-type cells incorporate exogenous thymine extremely poorly, but incorporate thymidine better. (2) Thethy−tlr+mutants are able to incorporate thymine only when high concentration are used, but can utilize a low concentration of thymidine. (3) Thethy−mutants are able to incorporate exogenous thymine as well as thymidine at low concentration. (4) Thetlrmutation is a thymine-specific one.

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