A comparison of the amino-terminal sequences of 3-deoxy-D-arabino -heptulosonate-7-phosphate synthase isoenzymes from Escherichia coli

Cloning of an aroF allele encoding a tyrosine-insensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase

In Escherichia coli, genes aroF+, aroG+, and aroH+ encode isoenzymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthases that are feedback inhibited by tyrosine, phenylalanine, and tryptophan, respectively. A single base pair change in aroF causes a Pro-148-to-Leu-148 substitution and results in a tyrosine-insensitive enzyme.

New prospects for deducing the evolutionary history of metabolic pathways in prokaryotes: aromatic biosynthesis as a case-in-point

Metabolic pathways of prokaryotes are more biochemically diverse than is generally recognized. Distinctive biochemical features are shared by phylogenetic clusters. The hierarchical levels of character-state clustering depends upon evolutionary events which fortuitously became fixed in the genome of a common ancestor. Prokaryotes can now be ordered on a phylogenetic tree. This allows the evolutionary steps that underlie the construction and regulation of appropriately complex biochemical pathways to be traced in an evolutionary progression of prokaryote types that house these pathways. Essentially the approach is to deduce ancestral character states at ever deeper phylogenetic levels, utilizing logical principles of maximum parsimony. The current perspective on the evolution of the biochemical pathway for biosynthesis of aromatic amino acids is developed as a case-in-point model for analyses that should be feasible with many major metabolic systems. Phenylalanine biosynthesis probably arose prior to the addition of branches leading to tyrosine and tryptophan. An evolutionary scenario is developed that begins with non-enzymatic reactions which may have operated in primitive systems, followed by the evolution of an enzymatic system that pre-dated the divergence of major lineages of modern eubacteria (Gram-positive bacteria, Gram-negative purple bacteria, and cyanobacteria).

The recent evolutionary origin of the phenylalanine-sensitive isozyme of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase in the enteric lineage of bacteria

Evolutionary events that generated the three regulatory isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase present in contemporary strains of Escherichia coli have been proposed recently [Ahmad et al. (1986) J Bacteriol 165:146-154]. The phylogenetic subdivision of gram-negative prokaryotes studied (Superfamily B) includes enteric bacteria, an Oceanospirillum cluster, pseudomonad Group I (e.g., Pseudomonas aeruginosa), pseudomonad Group V (e.g., Xanthomonas), and the Acinetobacter grouping. DAHP synthase-phe, a regulatory isozyme subject to allosteric control by L-phenylalanine, was the last member of the isozyme family to evolve. Thus, DAHP synthase-phe is absent throughout Superfamily B except within the enteric lineage. Bacteria that make up the enteric lineage (Escherichia, Klebsiella, Erwinia, Serratia, Proteus, Aeromonas, and Alteromonas) were examined in detail; DAHP synthase-phe was present in each of these organisms. Therefore, the isozyme originated between the separation of the enteric and Oceanospirillum lineages, prior to the divergence of Alteromonas putrefaciens (44% homology with E. coli by DNA:rRNA hybridization) from the rest of the enteric lineage. DAHP synthase-tyr and DAHP synthase-trp were uniformly present within the enteric lineage, although it was often necessary to derepress DAHP synthase-trp by physiological manipulation in order to demonstrate its presence.

Evolution of the regulatory isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase present in the Escherichia coli genealogy

The evolutionary history of isozymes for 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase has been constructed in a phylogenetic cluster of procaryotes (superfamily B) that includes Escherichia coli. Members of superfamily B that have been positioned on a phylogenetic tree by oligonucleotide cataloging possess one or more of four distinct isozymes of DAHP synthase. DAHP synthase-0 is insensitive to feedback inhibition, while DAHP synthase-Tyr, DAHP synthase-Trp, and DAHP synthase-Phe are sensitive to feedback inhibition by L-tyrosine, L-tryptophan, and L-phenylalanine, respectively. The evolutionary history of this isozyme family can be deduced within superfamily B by using a cladistic methodology of maximum parsimony (R. A. Jensen, Mol. Biol. Evol. 2:92-108, 1985). DAHP synthase-0 was found in Acinetobacter species and in Oceanospirillum minutulum, organisms that also possess DAHP synthase-Tyr. These two isozymes were apparently present in a common ancestor that predated the evolutionary divergence of contemporary superfamily B sublineages. DAHP synthase-0 is postulated to have been the evolutionary forerunner of DAHP synthase-Trp. The newly evolved DAHP synthase-Trp is postulated to have possessed sensitivity to feedback inhibition by chorismate as well as by L-tryptophan, chorismate sensitivity having been retained in rRNA group I pseudomonads (minor sensitivity), group V pseudomonads (very sensitive), and Lysobacter enzymogenes (ultrasensitive). Organisms constituting the enteric lineage of the phylogenetic tree (including a cluster of four Oceanospirillum species) have all lost the chorismate sensitivity of DAHP synthase-Trp. The absence of DAHP synthase-Phe in the Oceanospirillum cluster of organisms supports the previous conclusion that DAHP synthase-Phe evolved recently within superfamily B, being present only Escherichia coli and its close relatives.

Nucleotide sequence and transcription of the phenylalanine and tyrosine operons of Escherichia coli K12

A 4509 base-pair DNA fragment containing the phenylalanine and tyrosine operons of Escherichia coli K12 has been sequenced, and the pattern of transcription of these operons examined by S1 mapping, primer extension and galK fusion analyses. The phe operon consists of promoter, operator, leader region containing the phe attenuator and the pheA gene encoding chorismate mutase/prephenate dehydratase. The tyr operon consists of promoter, operator, a short leader region without an attenuator, and two structural genes aroF and tyrA encoding the tyrosine-sensitive isoenzyme of 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) synthetase and chorismate mutase/prephenate dehydrogenase, respectively. A bidirectional transcription terminator occurs between the two operons. The predicted amino acid sequences of chorismate mutase/prephenate dehydrogenase and chorismate mutase/prephenate dehydratase are homologous at their N termini, while the tyrosine-sensitive isoenzyme of DAHP synthetase is closely homologous to the phenylalanine-sensitive isoenzyme encoded by aroG.