Nalidixic acid-resistant mutants of Escherichia coli CGSC #6353 capable of growth at 48-degrees-C were obtained by mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine. Cotransductional analyses employing phage P1 indicated that the mutation resulting in the phenotype of growth at 48-degrees-C is an allele of the gyrA structural gene. Similar thermal inactivation kinetics were observed for ribosomes isolated from a thermotolerant (T/r) mutant grown at both 37-degrees-C and 48-degrees-C and from the parental strain grown at 37-degrees-C. Cell-free extracts prepared from the T/r mutant grown at 48-degrees-C exhibited a sharp increase in protein synthesis at 55-degrees-C, whereas this effect was not displayed by extracts from the mutant or parental strains grown at 37-degrees-C. In addition, preincubation at 55-degrees-C enhanced protein synthesis at 37-degrees-C up to 15-fold in an extract prepared from the T/r mutant grown at 48-degrees-C, whereas comparable values were 2.6- to 3.0-fold for extracts from the mutant and parental strains grown at 37-degrees-C.