a1 Kurchatov's Institute of Atomic Energy, Moscow, USSR
The 150 independently isolated thy− mutants of E. coli K12 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.
The thy locus is located between the cys and ser/gly genes, and is a linear structure where 134 thy mutants 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—tlr and td—were found. Mutations of tlr lead to a reduced thymine requirement (0·5 μg./ml. instead of 20 μg./ml.). A mutation of td results in thymidine sensitivity.
This latter character is expressed when the td-s allele is transferred into E. coli K12, 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 the thr gene. The most probable gene order is: tlr-td-thr.
The following results have been obtained from 14C-thymine incorporation experiments with wild-type cells, as well as with thy−tlr+ and thy−tlr− cells: (1) Wild-type cells incorporate exogenous thymine extremely poorly, but incorporate thymidine better. (2) The thy−tlr+ mutants are able to incorporate thymine only when high concentration are used, but can utilize a low concentration of thymidine. (3) The thy− mutants are able to incorporate exogenous thymine as well as thymidine at low concentration. (4) The tlr mutation is a thymine-specific one.
(Received March 12 1966)