THYMINELESS MUTATION SITE ON ESCHERICHIA COLI CHROMOSOME

Effect of the folic acid analogue, trimethoprim, on growth, macromolecular synthesis, and incorporation of exogenous thymine in Escherichia coli

The effect of trimethoprim [2,4-diamino-5(2',4',5'trimethoxybenzyl)-pyrimidine] in the presence of thymine on Escherichia coli B temperature-sensitive and non-temperature-sensitive Thy(') strains and a phosphodeoxyribomutase-negative mutant was studied. The inhibitory effect of 5 mug of trimethoprim per ml on the growth of E. coli B was not overcome by thymine, thymidine, or thymidylate even in the presence of one-carbon metabolites and related metabolites. Deoxyribonucleic acid (DNA) and protein synthesis were more severely inhibited than ribonucleic acid (RNA) synthesis. The inhibition of DNA synthesis was partially reversed by addition of deoxyadenosine to increase the incorporation of exogenous thymine. By contrast, the inhibition of protein was not reversed even with one-carbon metabolites present, in keeping with the requirement for formylmethionyl-transfer RNA(F) for initiation. However, the inhibition of both DNA and protein synthesis in a phosphodeoxyribomutase-negative strain by 1 mug of trimethoprim per ml with thymine present was partially reversed by deoxyadenosine and one-carbon metabolites, and nearly normal growth occurred. 5-Fluorodeoxyuridine added at the time of addition of trimethoprim prevented the inhibition. Sulfadiazine in the presence of thymine inhibited both Thy(+) and Thy(-) strains whereas trimethoprim (with thymine) did not inhibit Thy(-) organisms. The effect of trimethoprim on the incorporation of labeled thymine into DNA was also studied. These experiments support the concept that trimethoprim in conjunction with the action of thymidylate synthetase inhibits the growth of Thy(+) cells because of a depletion of tetrahydrofolate. DNA synthesis is inhibited initially by a limitation of thymine nucleotide precursor, resulting from the indirect inhibition of thymidylate synthetase and the poor incorporation of exogenous thymine.

A genetical study of thymineless mutants of E. coli K12

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.