Tryptophan synthetase levels in Escherichia coli, Shigella dysenteriae, and transduction hybrids

The tryptophan repressor sequence is highly conserved among the Enterobacteriaceae

Tryptophan biosynthesis in Escherichia coli is regulated by the product of the trpR gene, the tryptophan (Trp) repressor. Trp aporepressor binds the corepressor, L-tryptophan, to form a holorepressor complex, which binds trp operator DNA tightly, and inhibits transcription of the tryptophan biosynthetic operon. The conservation of trp operator sequences among enteric Gram-negative bacteria suggests that trpR genes from other bacterial species can be cloned by complementation in E. coli. To clone trpR homologues, a deletion of the E. coli trpR gene, delta trpR504, was made on a plasmid by site-directed mutagenesis, then crossed onto the E. coli genome. Plasmid clones of the trpR genes of Enterobacter aerogenes and Enterobacter cloacae were isolated by complementation of the delta trpR504 allele, scored as the ability to repress beta-galactosidase synthesis from a prophage-borne trpE-lacZ gene fusion. The predicted amino acid sequences of four enteric TrpR proteins show differences, clustered on the backside of the folded repressor, opposite the DNA-binding helix-turn-helix substructures. These differences are predicted to have little effect on the interactions of the aporepressor with tryptophan, holorepressor with operator DNA, or tandemly bound holorepressor dimers with one another. Although there is some variation observed at the dimer interface, interactions predicted to stabilize the interface are conserved. The phylogenetic relationships revealed by the TrpR amino acid sequence alignment agree with the results of others.

Naturally occurring promoter down mutation: nucleotide sequence of the trp promoter/operator/leader region of Shigella dysenteriae 16

The promoter/operator/leader region of the trp operon of Shigella dysenteriae 16 has single base pair differences from the corresponding region of Escherichia coli at positions -24 and -13. The difference at -13 was shown to be responsible for the 90% reduction in promoter function characteristic of the trp operon of S. dysenteriae. The base pair difference at position -13 also renders the operator partially constitutive. This allows the organism to maintain relatively high repressed levels of the trp enzymes and increases the relative importance of attenuation as a transcription control mechanism. These findings and the earlier observation that the trpE protein of S. dysenteriae is only slightly active explain the low in vivo expression of the trp operon of this organism. Nutritional studies suggest that operons involved in other amino acid biosynthetic pathways in S. dysenteriae 16 may be similarly partially inactivated.

Tryptophan operon regulation in interspecific hybrids of enteric bacteria

We examined tryptophan regulation in merodiploid hybrids in which a plasmid carrying the trp operon of Escherichia was introduced into Trp mutants of other enteric genera, or in which a plasmid carrying the trpR+ (repressor) gene of E. coli was transfered into fully constitutive trpR mutants of other genera. In these hybrids the trp operon of one species is controlled by the repressor of a different species. Similar investigations were possible in transduction hybrids in which either the trp operon or the trpR+ locus of Shigella dysenteriae was introduced into E. coli. Our measurements of trp enzymes levels in repressed and nonrepressed cells indicate that Trp regulation is normal, with only minor quantitative variations, in hybrids between E coli and Shigella dysenteriae, Salmonella typhimurium, Klebsiella aerogenes, Serratia marcescens, and Proteus mirabilis. Our results support the idea that a repressor-operator mechanism for regulating trp messenger ribonucleic acid production evolved in a common ancestor of the enteric bacteria, and that this repressor-operator recognition has been conversed during the evolutionary divergence of the Enterobacteriaceae.

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

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 maps outside the structural gene region and is closely linked to the S. dysenteriae trp operator. trpP (S) reduced the rate of trp messenger ribonucleic acid synthesis, and consequently trp enzyme levels, 10-fold relative to wild-type E. coli. In recombinants in which the structural genes of E coli were under the control of the S. dysenteriae promoter, enzyme levels were also reduced 10-fold. In some fast-growing revertants of the original hybrids, the rates of trp messenger ribonucleic acid synthesis and levels of tryptophan synthetase were restored to values characteristic of wild-type E.coli. Thus, the Trp auxotrophy associated with the S dysenteriae trp operon derives from the combination of a defective ASase and decreased expression of the entire operon imposed by trpP (S).

Characterization of mutants with single and multiple defects in the tryptophan biosynthetic pathway in Bacillus subtilis

Sixty-five tryptophan auxotrophs which map in a cluster on the genome of Bacillus subtilis were characterized on the basis of (i) growth response, (ii) accumulation of intermediate compounds, and (iii) determination of enzymatic defects. They could be placed into six phenotypic classes. Certain of the mutants exhibited pleiotropic effects on more than one enzymatic activity in a manner different from those effects reported for the tryptophan pathway in other organisms. Invariably, mutations in the second gene, that coding for phosphoribosyl transferase activity, were found to lack the indoleglycerol phosphate synthase activity specified by the third gene in the cluster; however, this polarity did not extend to genes more distal in the cluster. Furthermore, mutations in the gene which codes for phosphoribosyl-anthranilate isomerase not only led to a loss of this enzyme but also to a loss of phosphoribosyl transferase and indoleglycerol phosphate synthase. In contrast, mutations in either of the loci coding for these latter functions had no apparent effect on isomerase activity. No polarity of the conventional type was found, e.g., none of the mutations in any gene led to polarized effects on the levels of the enzymes specified by the other genes of the cluster. These observations indicated a possible in vivo aggregation involving the transferase, isomerase, and synthase enzymes, with the isomerase acting as the "key" enzyme in the aggregate.

Comparison of the tryptophan synthetase alpha-subunits of several species of Enterobacteriaceae

Creighton, T. E. (Stanford University, Stanford), D. R. Helinski, R. L. Somerville, and C. Yanofsky. Comparison of the tryptophan synthetase alpha subunits of several species of Enterobacteriaceae. J. Bacteriol. 91:1819-1826. 1966.-The tryptophan synthetase alpha subunits of Escherichia coli K-12, E. coli B, Shigella dysenteriae, Salmonella typhimurium, and Aerobacter aerogenes have been purified and their structures compared. Each of these alpha subunits exhibits a sedimentation coefficient of about 2.7S. Peptide patterns of trypsin plus chymotrypsin digests of the alpha subunits have indicated that all of the alpha subunits have peptide regions in common. The patterns of E. coli K-12, E. coli B, and S. dysenteriae alpha subunits appear to be nearly identical, whereas the alpha subunits from S. typhimurium and A. aerogenes differ from those of E. coli and from each other. It has also been shown that the E. coli structural gene for the alpha subunit is translated identically in E. coli and S. typhimurium.