Studies on the mutability of Escherichia coli K12 : I. Suppression and high spontaneous mutation in a threonine auxotroph

Abstract

Strain TH 1014 of Escherichia coli K12 Hfr H is a leaky mutant requiring l-threonine for growth and has a very high rate of spontaneous reversion to prototrophy. Through conjugation experiments with appropriate F − strains, it was found that the leakiness in this strain was due to the existence of a gene ( mod) locating near the leu locus and modifying a threonine deficiency ( thr 1014) at the usual threonine site and the reversions to complete prototrophy took place at a site ( fgr) residing between the gal and try loci. These genetic sites regarding threonine reversion of the strain TH 1014 were transferred by conjugation into F − strains. Studies with these reconstructed female strains, as well as those with original male strains, showed that strains carrying thr 1014 mod + and fgr spontaneously mutated, frequently in the order of 10 −4, but those carrying thr 1014 mod and fgr did not. These studies also indicated that, at least in a few revertant strains examined, the co-existence of mod + and fgr + together with thr 1014 might be necessary for active growth in the absence of threonine. Studies on the growth of mutants of T4 carrying the amber or ochre nonsence codons suggested that neither the mod + nor the fgr + gene is a suppresor for these nonsense codons.