Internal homologies in the two aspartokinase-homoserine dehydrogenases of Escherichia coli K-12

A novel bifunctional aspartate kinase-homoserine dehydrogenase from the hyperthermophilic bacterium, Thermotoga maritima

The orientation of the three domains in the bifunctional aspartate kinase-homoserine dehydrogenase (AK-HseDH) homologue found in Thermotoga maritima totally differs from those observed in previously known AK-HseDHs; the domains line up in the order HseDH, AK, and regulatory domain. In the present study, the enzyme produced in Escherichia coli was characterized. The enzyme exhibited substantial activities of both AK and HseDH. l-Threonine inhibits AK activity in a cooperative manner, similar to that of Arabidopsis thaliana AK-HseDH. However, the concentration required to inhibit the activity was much lower (K0.5 = 37 μM) than that needed to inhibit the A. thaliana enzyme (K0.5 = 500 μM). In contrast to A. thaliana AK-HseDH, Hse oxidation of the T. maritima enzyme was almost impervious to inhibition by l-threonine. Amino acid sequence comparison indicates that the distinctive sequence of the regulatory domain in T. maritima AK-HseDH is likely responsible for the unique sensitivity to l-threonine.

Abbreviations: AK: aspartate kinase; HseDH: homoserine dehydrogenase; AK–HseDH: bifunctional aspartate kinase–homoserine dehydrogenase; AsaDH: aspartate–β–semialdehyde dehydrogenase; ACT: aspartate kinases (A), chorismate mutases (C), and prephenate dehydrogenases (TyrA, T).

Mechanism of control of Arabidopsis thaliana aspartate kinase-homoserine dehydrogenase by threonine

The regulatory domain of the bifunctional threonine-sensitive aspartate kinase homoserine dehydrogenase contains two homologous subdomains defined by a common loop-alpha helix-loop-beta strand-loop-beta strand motif. This motif is homologous with that found in the two subdomains of the biosynthetic threonine-deaminase regulatory domain. Comparisons of the primary and secondary structures of the two enzymes allowed us to predict the location and identity of the amino acid residues potentially involved in two threonine-binding sites of Arabidopsis thaliana aspartate kinase-homoserine dehydrogenase. These amino acids were then mutated and activity measurements were carried out to test this hypothesis. Steady-state kinetic experiments on the wild-type and mutant enzymes demonstrated that each regulatory domain of the monomers of aspartate kinase-homoserine dehydrogenase possesses two nonequivalent threonine-binding sites constituted in part by Gln(443) and Gln(524). Our results also demonstrated that threonine interaction with Gln(443) leads to inhibition of aspartate kinase activity and facilitates the binding of a second threonine on Gln(524). Interaction of this second threonine with Gln(524) leads to inhibition of homoserine dehydrogenase activity.

Homology of lysS and lysU, the two Escherichia coli genes encoding distinct lysyl-tRNA synthetase species

In Escherichia coli, two distinct lysyl-tRNA synthetase species are encoded by two genes: the constitutive lysS gene and the thermoinducible lysU gene. These two genes have been isolated and sequenced. Their nucleotide and deduced amino acid sequences show 79% and 88% identity, respectively. Codon usage analysis indicates the lysS product being more efficiently translated than the lysU one. In addition, the lysS sequence exactly coincides with the sequence of herC, a gene which is part of the prfB-herC operon. In contrast to the recent proposal of Gampel and Tzagoloff (1989, Proc. Natl. Acad. Sci. USA 86, 6023-6027), the lysU sequence is distinct from the open reading frame located adjacent to frdA, although large homologies are shared by these two genes.

Nucleotide sequence of aceK, the gene encoding isocitrate dehydrogenase kinase/phosphatase

In Escherichia coli, the phosphorylation and dephosphorylation of isocitrate dehydrogenase (IDH) are catalyzed by a bifunctional protein kinase/phosphatase. We have determined the nucleotide sequence of aceK, the gene encoding IDH kinase/phosphatase. This gene consists of a single open reading frame of 1,734 base pairs preceded by a Shine-Dalgarno ribosome-binding site. Examination of the deduced amino acid sequence of IDH kinase/phosphatase revealed sequences which are similar to the consensus sequence for ATP-binding sites. This protein did not, however, exhibit the extensive sequence homologies which are typical of other protein kinases. Multiple copies of the REP family of repetitive extragenic elements were found within the intergenic region between aceA (encoding isocitrate lyase) and aceK. These elements have the potential for combining to form an exceptionally stable stem-loop structure (delta G = -54 kcal/mol [ca. -226 kJ/mol]) in the mRNA. This structure, which masks the ribosome-binding site and start codon for aceK, may contribute to the downshift in expression observed between aceA and aceK. Another potential stem-loop structure (delta G = -29 kcal/mol [ca. 121 kJ/mol]), unrelated to the REP sequences, was found within aceK.

Methionine biosynthesis in Enterobacteriaceae: biochemical, regulatory, and evolutionary aspects

The genes coding for the enzymes involved in methionine biosynthesis and regulation are scattered on the Escherichia coli chromosome. All of them have been cloned and most have been sequenced. From the information gathered, one can establish the existence (upstream of the structural genes coding for the biosynthetic genes and the regulatory gene) of "methionine boxes" consisting of two or more repeats of an octanucleotide sequence pattern. The comparison of these sequences allows the extraction of a consensus operator sequence. Mutations in these sequences lead to the constitutivity of the vicinal structural gene. The operator sequence is the target of a DNA-binding protein--the methionine aporepressor--which has been obtained in the pure state, for which S-adenosylmethionine acts as the corepressor. Mutations in the corresponding gene lead to the constitutive expression of all the methionine structural genes. The physicochemical properties of the methionine aporepressor are being investigated.

Complete nucleotide and deduced amino acid sequence of bovine phenylethanolamine N-methyltransferase: partial amino acid homology with rat tyrosine hydroxylase

We report here the isolation of a cDNA clone containing the full coding region of bovine phenylethanolamine N-methyltransferase (PNMTase, EC 2.1.1.28, S-adenosyl-L-methionine:phenylethanolamine N-methyltransferase). The complete nucleotide sequence of the cDNA has been determined, and the amino acid sequence of PNMTase deduced. Cultured cells transfected with an expression vector containing this cDNA produced high levels of PNMTase enzymatic activity. Antibodies specific for tyrosine hydroxylase [EC 1.14.16.2, tyrosine 3-monooxygenase; L-tyrosine, tetrahydrobiopterine: oxygen oxidoreductase (3-hydroxylating)], the first enzyme in the catecholamine pathway, possess a striking affinity for the PNMTase protein synthesized in vitro. Comparison of the deduced amino acid sequence of bovine PNMTase to rat tyrosine hydroxylase reveals that PNMTase shares significant homology with tyrosine hydroxylase and supports previous protein and immunological data suggesting that the catecholamine biosynthetic enzymes are structurally related.

L-asparaginase genes in Escherichia coli: isolation of mutants and characterization of the ansA gene and its protein product

Mutants of Escherichia coli have been isolated which are resistant to beta-aspartyl hydroxamate, a lethal substrate of asparaginase II in fungi and a substrate for asparaginase II in E. coli. Among the many phenotypic classes observed, a single mutant (designated GU16) was found with multiple defects affecting asparaginases I and II and aspartase. Other asparaginase II-deficient mutants have also been derived from an asparaginase I-deficient mutant. The mutant strain, GU16, was unable to utilize asparagine and grew poorly on aspartate as the sole source of carbon; transformation of this strain with an E. coli recombinant plasmid library resulted in a large recombinant plasmid which complemented both these defects. Two subclones were isolated, designated pDK1 and pDK2; the former complemented the partial defect in the utilization of aspartate, although its exact function was not established. pDK2 encoded the asparaginase I gene (ansA), the coding region of which was further defined within a 1.7-kilobase fragment. The ansA gene specified a polypeptide, identified in maxicells, with a molecular weight of 43,000. Strains carrying recombinant plasmids encoding the ansA gene overproduced asparaginase I approximately 130-fold, suggesting that the ansA gene might normally be under negative regulation. Extracts from strains overproducing asparaginase I were electrophoresed, blotted, and probed with asparaginase II-specific antisera; no cross-reaction of the antisera with asparaginase I was observed, indicating that asparaginases I and II are not appreciably related immunologically. When a DNA fragment containing the ansA gene was used to probe Southern blots of restriction endonuclease-digested E. coli chromosomal DNA, no homologous sequences were revealed other than the expected ansA-containing fragments. Therefore, the genes encoding asparaginases I and II are highly sequence related.