Naturally occurring sites within the Shigella dysenteriae tryptophan operon severely limit tryptophan biosynthesis

Abstract

We investigated the structural, functional, and regulatory properties of the Shigella dysenteriae tryptophan (trp) operon in transduction hybrids in which the cysB-trp region of Escherichia coli is replaced by the corresponding region from S. dysenteriae. Tryptophan biosynthesis was largely blocked in the hybrids, although the order of the structural genes was identical with that of E. coli. Nutritional tests and enzyme assays revealed that the hybrids produced a defective anthranilate synthetase (ASase). Deletion mapping identified two distinct sites in trpE, each of which was partially responsible for the instability and low activity of ASase. We also discovered a pleiotropic site trpP(S) that

Tryptophan operon regulation in interspecific hybrids of enteric bacteria

“…volume of crude extracts of the Trp mutant recipients also grown under derepressing conditions. The mixed extracts were incubated at 37 C, and the activity of the enzyme contrib-uted uniquely by E. coli MM1 (either ASase or PRTase) was assayed at hourly intervals. There was no significant difference in the decay of activity in any of the mixed extracts relative to the E. coli MM1 control (data not shown).…”

“…Construction of S. dysenteriae-E. coli transduction hybrids. Plkc transduction with phage grown on S. dysenteriae Shl6 was done basically as described by Yanofsky and Lennox (53), with the modifications of Manson and Yanofsky (37). The interspecific transduction frequency with E. coli was very low, but an adequate number of transductants was obtained by having 109 cells per plate and by using a phage-to-bacteria ratio of 5.…”

“…The preparation of crude cell extracts and the assays for anthranilate synthetase (glutamine-stimulated activity), phosphoribosyl anthranilate transferase (PRTase), and TSase a and P2 (indole-totryptophan activity) have been described (37). Protein was determined by the method of Lowry et al (31).…”

Silent genes in prokaryotes

“…Consideration of auxotrophy in Shigella dysenteriae [43,44] further emphasises the difficulty of distinguishing between cryptic and silent genes. S. dysenteriae is apparently unable to synthesise at least 12 amino acids.…”

“…Is the silent trp operon destined for deletion, or is it retained due to a periodic requirement for tryptophan biosynthesis? The partial function of the Shigella trp operon, the different ecology of E. coli and Shigella [see 43], and the revertibility to Trp +, argue for a functional or a cryptic operon. On this view, it may be retained in the genome due to a periodic requirement for tryptophan, perhaps when it is a transient resident of the vertebrate gut, whereupon an outgrowth of Trp + revertants occurs.…”