Genetic studies of the lac repressor

Isolation of a Mutant Auxotrophic forL-Alanine and Identification of Three Major Aminotransferases That SynthesizeL-Alanine inEscherichia coli

“…To test this possibility, we determined the nucleotide sequence of the yfdZ genes of four independent alanine auxotrophs. All the mutants tested appeared to possess a nucleotide change from guanine to adenine in the open reading frame of the yfdZ gene, in good agreement with a characteristic feature of NTG, that of causing a preferential base pair change from G-C to A-T. 35 The minimal medium contained (A) pyridoxine (75 ng/mL), (B) pyridoxine (75 ng/mL) and L-alanine (20 mg/mL), (C) pyridoxine (75 ng/mL) and L-serine (300 mg/mL), and (D) pyridoxine (75 ng/mL), serine (300 mg/mL), and L-alanine (20 mg/mL). Strains: 1, HYE031 (avtA, yfbQ, serC); 2, HYE008 (avtA, yfbQ, Ala À ); 3, HYE021 (avtA, yfbQ); and 4, W3110 (wild type).…”

A kirromycin resistant elongation factor EF‐Tu from Escherichia coli contains a threonine instead of an alanine residue in position 375

“…Apparently the substitution of Ala-375 by Thr (ACA) is caused by a transition (G-C ~ A-T), in accordance with the fact that ethyl methane sulphonate was used originally for mutagenesis [10,30,31 ]. Probably the mutation of EF-TuA R is restricted to this transition although we have not analyzed the tryptic peptides T7, T14, T16, T18, T23 and T24 (cf.…”

Amino acid substitutions that reduce the affinity of penicillin‐binding protein 3 of Escherichia coli for cephalexin

“…Studies on the generation of nonsense mutations in the lac1 gene have shown that > 99% of the lesions produced by these mutagens occur at G . C base pairs [36] whereas the mutations that produced the class 1 -3 mutants were at A T base pairs. Presumably there are no mutations at G 1 C base pairs within the pbpB gene that give rise to an alteration of PBP 3 that produces a more than sevenfold increase in resistance to cephalexin and the mutations that we observed are due to the ability of EMS and NQO to cause lesions at A .…”

“…In all cases mutants that appeared to be identical in their phenotype (i. e. in their pattern of cross-resistance to p-lactam antibiotics) were found to have mutations that produced the identical amino acid substitution in PBP 3. All possible single nucleotide changes should be obtained with ultraviolet mutagenesis [36] and the failure to find additional classes of mutants following this treatment suggests that there were probably few, if any, other single nucleotide changes in the pbpB gene that produced this phenotype. The amino acid substitutions that can occur by single nucleotide changes are limited by the genetic code and amino acid substitutions at other positions in PBP 3, or different amino acid substitutions at the observed positions, might produce the cephalexin-resistant phenotype if double nucleotide changes could be obtained readily.…”