Genetic analysis of thymineless(thy) mutants in Salmonella typhimurium.

MINOPTERIN may be used to select readily thymineless ( t h y ) mutants in A Escherichia coli ( OKADA, HOMMA, and SONOHARA 1962) ; however, this selection is more difficult in Salmonella typhimuriun strain LT2. Since this may be due to the rarity of thy mutation in this organism, the mutagen N-methy1-N'nitro-N-nitrosoguanidine (NG) was combined with aminopterin to successfully mutate and select thy mutants (NISHIOKA and EISENSTARK 1964). Approximately 300 t hy mutants of S. typhimurium were collected (Table 1 ) and many of these were analyzed, seeking answers to these questions: ( 1 ) What is the precise position of thy on the chromosome map of S. typhimurium (see SANDERSON and DEMEREC 1965 for detailed map)? (2) What loci are included in the same phage P22 transducing fragment with thy? (3) What is the clockwise map orientation of the thy locus and its neighbors? (4) Do all thy fall into a single locus? (5) Are all thy physiologically identical? (6) By reversion tests with a series of mutagens, which thy appear to be single-site and which multisite mutants? In the course of analyses, a number of observations led to additional probing of the degree of heterogeneity of transducing particles, sensitivity of cells to deoxyribosides, and the mapping of neighbors of thy. While this investigation was in progress, a report appeared (ALIKHANIAN et al. 1966) of similar experiments with E. coli.