Elective production of thymine-less mutants.

Abstract

THYMINE-LESS (thy −) mutants of micro-organisms have not been easy to obtain. They die rapidly in the absence of thymine1 so that, when penicillin is used to concentrate them, none survives the treatment. To discover them after mutagenic treatment requires direct detection methods and this, in turn, either good fortune2,3 or a very high frequency of thy − mutants among the survivors4. We have recently discovered a method whereby thy − mutants of some strains of Escherichia coli 15 and K 12 can be produced in frequencies of the order of 50 per cent. Although no success was obtained with Salmonella typhimurium, the method seems to involve the general principle of differential growth-rates. With E. coli both the thy − and thy + strains are inhibited by aminopterin, but it seems that only in the thy − can thymidine completely overcome that inhibition.