Transduction and recombination study of linkage relationships among the genes controlling tryptophan synthesis in Escherichia coli

Abstract

The relationship between gene sequence and the sequence of biochemical reactions in the biosynthesis of tryptophan was investigated in the K12 strain of Escherichia coli. Transduction with phage P1 kc was employed for genetic mapping. All the tryptophan auxotrophs examined were found to be mutant in a small region of genetic material (>80% joint transduction of the most distal markers). One strain with a block in the synthesis of several aromatic compounds, including tryptophan, was found to be mutant at a site weakly linked to the other tryp genes. Two cysteine auxotrophs were also mutant at sites linked to the tryp region. The transduction tests employed suggested the following order of the tryp genes and the cysteine markers: cys … tryp 4. tryp 3. tryp 1. tryp 2. This order differs from the gene order established with the corresponding mutants of Salmonella typhimurium only with respect to the last two tryp genes, tryp 1 and tryp 2. However, since the proteins controlled by these genes are both involved in the terminal reaction in tryptophan synthesis, and thus cannot be assigned a biosynthetic order, the results obtained indicate that the clustered tryp genes of E. coli are arranged in the same relative order as the steps in the biosynthesis of tryptophan which they control. An examination of the category of tryptophan auxotrophs which acquired a requirement for tryptophan simultaneously with mutation to resistance to phage T1 suggested that these strains arose as a result of deletions including the V 1 r locus and one or more tryp genes. It was found that in strain B, where mutations of this type are fairly frequent, most of the V 1 r tryp mutants lack all the clustered tryp genes. In strain K12, where V 1 r tryp mutants are rare among V 1 r isolates, the few strains obtained did not lack all the clustered tryp genes. Studies with hybrids formed between strains K12 and B suggested that genic material at or near the tryp region in the two strains determines the type of mutation pattern when cells are exposed to T1. The deletion from one V 1 r tryp mutant was transferred by transduction and 60% of the phage carrying the tryp 4 marker were found to carry the entire tryp region and thus were capable of converting tryp deletion mutants to tryptophan-independent strains.