The Isolation and Properties of Amino Acid Requiring Mutants of a Thymineless Bacterium

A strain of Escherichia coli requiring thymine, strain 15T-, loses the ability to multiply under conditions in which thymine is absent from its growth medium (Cohen and Barner, 1954). Pre-liminary studies had revealed that the "death" of the organism required the metabolism of a carbon source and concomitant active biosyn-theses. The hypothesis was proposed that death was a consequence of an inhibition of deoxyribo-nucleic acid (DNA) synthesis under conditions of continuing cytoplasmic synthesis, i. e., unbal-anced growth. As an approach to the further dissection of the metabolic requirements of the death of this organism, a number of mutants of strain 15T- were isolated, possessing nutritional