A cis/trans Test of the Effect of the First Enzyme for Histidine Biosynthesis on Regulation of the Histidine Operon

Previous studies showed that when triazolalanine was added to a derepressed culture of a histidine auxotroph, repression of the histidine operon occurred as though histidine had been added (6). However, when triazolalanine was added to a derepressed culture of a strain with a mutation in the first gene of the histidine operon which rendered the first enzyme for histidine biosynthesis resistant to inhibition by histidine, repression did not occur. The studies reported here represent a cis/trans test of this effect of mutations to feedback resistance. Using specially constructed merodiploid strains, we were able to show that the wild-type allele is dominant to the mutant (feedback resistant) allele and that the effect operates in trans. We conclude that the enzyme encoded by the first gene of the histidine operon exerts its regulatory effect on the operon not by acting locally at its site of synthesis, but by acting as a freely diffusible protein.     

trans-Recessive mutation in the first structural gene of the histidine operon that results in constitutive expression of the operon.

The first enzyme for histidine biosynthesis, encoded in the hisG gene, is involved in regulation of expression of the histidine operon in Salmonella typhimurium. The studies reported here concern the question of how expression of the histidine operon is affected by a mutation in the hisG gene that alters the allosteric site of the first enzyme for histidine biosynthesis, rendering the enzyme completely resistant to inhibition by histidine. The intracellular concentrations of the enzymes encoded in the histidine operon in a strain carrying such a mutation on an episome and missing the chromosomal hisG gene are three- to fourfold higher than in a strain carrying a wild-type hisG gene on the episome. The histidine operon on such a strain fails to derepress in response to histidine limitation and fails to repress in response to excess histidine. Furthermore, utilizing other merodiploid strains, we demonstrate that the wild-type hisG gene is trans dominant to the mutant allele with respect to this regulatory phenomenon. Examination of the regulation of the histidine operon in strains carrying the feedback-resistant mutation in an episome and hisT and hisW mutations in the chromosome showed that the hisG regulatory mutation is epistatic to the hisT and hisW mutations. These data provide additional evidence that the first enzyme for histidine biosynthesis is involved in autogenous regulation of expression of the histidine operon.     

Interaction Between the First Enzyme for Histidine Biosynthesis and Histidyl Transfer Ribonucleic Acid

Previous studies suggested that phosphoribosyltransferase, which catalyzes the first step of the pathway for histidine biosynthesis in Salmonella typhimurium and which is sensitive to inhibition by histidine, plays a role in repression of the histidine operon. Recently, we showed that the enzyme has a high affinity for histidyl transfer ribonucleic acid (His-tRNA), which is known to participate in the repression process. In the present study, we have investigated further the interaction between the enzyme and His-tRNA. We found that His-tRNA binds at a site on phosphoribosyltransferase distinct from the catalytic site and the histidine-sensitive site; that the substrates of the enzyme inhibit the binding of His-tRNA, whereas histidine does not do so; that, once a complex has been formed between phosphoribosyltransferase and His-tRNA, the substrates of the enzyme decrease the stability of the complex, whereas histidine is without effect; and that purified phosphoribosyltransferase which has a defect in its inhibition by histidine (produced by mutation) displays an altered ability to bind His-tRNA, a finding which may be a reflection of the fact that mutants producing such a defective enzyme display an alteration of the repression process.     

Duplications of histidine transport genes in Salmonella typhimurium and their use for the selection of deletion mutants.

We demonstrate that tandem duplications of the histidine transport operon can be selected by requesting elevated levels of transport activity to be present. Several strains were constructed which contain duplications heterozygotic for either hisJ, hisQ, or hisP. The size of one duplication which was analyzed in detail is about 16 genes, with one end close to the promoter site (dhuA) of the histidine transport operon and, therefore, enclosing about 12 more genes counterclockwise to this operon. Duplication-carrying strains could be utilized for the selection of deletion mutations by requiring both copies of the operon to be rendered defective simultaneously and, therefore, unable to transport into the cell an inhibitory histidine analog, alpha-hydrazino imidazole propionic acid. Over 60% (probably as high as 100%) of the alpha-hydrazino imidazole propionic acid-resistant strains arising in the selection are deletion mutants. The principle of our selection method is generally applicable and will be useful in the accumulation of deletions for mapping and fusing of genes and other purposes.     

Histidine auxotrophy in commensal and disease-causing nontypeable Haemophilus influenzae

Histidine biosynthesis is one of the best studied metabolic pathways in bacteria. Although this pathway is thought to be highly conserved within and between bacterial species, a previous study identified a genetic region within the histidine operon (his) of nontypeable strains of Haemophilus influenzae (NTHI) that was more prevalent among otitis media strains than among throat commensal NTHI strains. In the present study, we further characterized this region and showed that genes in the complete his operon (hisG, -D, -C, -NB, -H, -A, -F, and -IE) are >99% conserved among four fully sequenced NTHI strains, are present in the same location in these four genomes, and are situated in the same gene order. Using PCR and dot blot hybridization, we determined that the his operon was significantly more prevalent in otitis media NTHI strains (106/121; 87.7%) than in throat strains (74/137; 54%) (prevalence ratio, 1.62; P<0.0001), suggesting a possible role in middle ear survival and/or acute otitis media. NTHI strains lacking the his operon showed attenuated growth in histidine-restricted media, confirming them as his-negative auxotrophs. Our results suggest that the ability to make histidine is an important factor in bacterial growth and survival in the middle ear, where nutrients such as histidine may be found in limited amounts. Those isolates lacking the histidine pathway were still able to survive well in the throat, which suggests that histidine is readily available in the throat environment.     

Two Mutations in the First Gene of the Histidine Operon of Salmonella typhimurium Affecting Control

Two strains with mutations in the first structural gene of the histidine operon of Salmonella typhimurium were characterized. (The first structural gene specifies the first enzyme of histidine biosynthesis, phosphoribosyltransferase, which is sensitive to feedback inhibition by histidine.) One mutation, hisG3934, results in a phosphoribosyltransferase which is no longer sensitive to feedback inhibition by histidine but is instead subject to inhibition by aspartic acid. The other mutation, hisG3935, allows the histidine operon to be partially repressed by several amino acids, including aspartic acid. Analysis of hisG3935 is consistent with the hypothesis that phosphoribosyltransferase is directly involved in the regulation of the histidine operon.     

Histidine regulation in Salmonella typhimurium: XVI. A sensitive radiochemical assay for histidinol dehydrogenase

A sensitive radiochemical assay for measurement of histidinol dehydrogenase is presented. The method is based upon separation of the product of the reaction. [¹⁴C]histidine, from the substrate, [¹⁴C]histidinol, on small Dowex 50 columns. The assay can be performed on cell extracts or on toluenized cells and is approximately 100 times more sensitive than previously reported assays for this enzyme.[¹⁴C]histidinol is obtained in high yields through conversion of uniformly labeled ¹⁴C-glucose by a strain of Salmonella typhimurium derepressed for the histidine operon and blocked at the histidinol dehydrogenase step. Accumulated [¹⁴C]histidinol is purified from the culture supernatant by ion-exchange chromatography.This sensitive assay has facilitated measurement of reduced levels of histidine operon expression in promoter mutants, and has been adapted for study of histidine operon regulation in a cell free protein synthesizing system.     

Histidyl-transfer ribonucleic acid synthetase mutants requiring a high internal pool of histidine for growth

Mutants that require histidine due to an altered structural gene for the histidyl-transfer ribonucleic acid synthetase (hisS) have been isolated by a general selection for histidine-requiring strains in which the mutation producing histidine auxotrophy is unlinked to the histidine operon. One of the mutants has been shown to require an abnormally high internal histidine pool for growth owing to an altered synthetase that is unstable at low histidine concentrations. It is difficult to determine accurately the K(m) for histidine of the synthetase enzyme from the mutant because of the instability of the enzyme at limiting histidine concentrations; however, a histidine K(m) value has been estimated that is approximately 100 times higher than the histidine K(m) of the wild-type enzyme. For the mutant strains to achieve the high internal pool of histidine required for growth, all the systems that transport histidine from the growth medium must be functioning to capacity. Amino acids that interfere with histidine transport strongly inhibit the growth of the mutants. The mutants have been useful in providing a selective genetic marker for transductional mapping in the hisS region. The mutants are discussed as representative of a general class of curable mutants that have an altered enzyme with poor affinity for a substrate or coenzyme.     

A Biochemical Characterization of Histidine Auxotrophs of Corynebacterium glutamicum

Two imidazoleglycerol (IG)-producing mutants and one imidazoleacetol (IA)-producing mutant were selected out of 14 histidine auxotrophs of C. glutamicum, by means of paper-chromatographic analysis of the culture broths of these mutants. Three of the histidine biosynthetic enzymes were determined for these mutants and a previously isolated histidinol-producing mutant of C. glutamicum. The IA-producing mutant and the l-histidinol-producing mutant had a defect in histidinol phosphate aminotransferase and histidinol dehydrogenase, respectively. IG-producers were not defective in these enzymes. These results were consistent with the histidine biosynthetic sequence known in other microorganisms.     

Histidine and Aromatic Permeases of Salmonella typhimurim

Mutants defective either in the histidine permease (hisP) or in the aromatic permease (aroP) were isolated in Salmonella typhimurium and were characterized. The hisP locus had a 49% linkage to purF by phage transduction. The aroP locus was close to proA. Merozygotes diploid for the hisP gene were constructed by episomal transfer, and hisP(+) was dominant over hisP. The properties of merozygotes are described and discussed. A method for the selection of revertants of hisP mutants was devised. By this method, one of the hisP mutants was characterized as an amber mutant. The specificity of the aromatic permease was investigated by using as substrates analogues of the aromatic amino acids and of histidine.     

Repression of 3-deoxy-D-arabinoheptulosonic acid-7-phosphate synthetase (trp) and enzymes of the tryptophan pathway in Escherichia coli K-12

Mutant strains of Escherichia coli K-12 have been isolated in which the synthesis of 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase (trp) is partially constitutive. The mutation causing derepression is closely linked to aroH [the structural gene for DAHP synthetase (trp)] and occurs in a locus designated aroJ. The aroJ mutation is not recessive in an aroJ(+)/aroJ(-) diploid strain, as the synthesis of DAHP synthetase (trp) is still derepressed in this strain. On the basis of its close linkage to aroH and its continued expression in an aroJ(+)/aroJ(-) diploid, it is postulated that aroJ is an operator locus controlling the expression of the structural gene aroH. In support of this conclusion, the synthesis of anthranilate synthetase is still normally repressible in aroJ(-) strains, whereas, in trpR(-) strains, both DAHP synthetase (trp) and anthranilate synthetase are synthesized constitutively. The synthesis of DAHP synthetase (trp) remains repressible in an operator-constitutive mutant of the tryptophan operon. In two trpS mutants which possess defective tryptophanyl transfer ribonucleic acid synthetase enzymes, neither DAHP synthetase (trp) nor anthranilate synthetase derepress under conditions in which the defective synthetase causes a decrease in growth rate. On the other hand, an effect of the trpS mutant alleles on the level of anthranilate synthetase has been observed in strains which are derepressed for the synthesis of this enzyme, because of a mutation in the gene trpR. Possible explanations for this effect are presented.     

Histidine biosynthesis genes in Lactococcus lactis subsp. lactis.

The genes of Lactococcus lactis subsp. lactis involved in histidine biosynthesis were cloned and characterized by complementation of Escherichia coli and Bacillus subtilis mutants and DNA sequencing. Complementation of E. coli hisA, hisB, hisC, hisD, hisF, hisG, and hisIE genes and the B. subtilis hisH gene (the E. coli hisC equivalent) allowed localization of the corresponding lactococcal genes. Nucleotide sequence analysis of the 11.5-kb lactococcal region revealed 14 open reading frames (ORFs), 12 of which might form an operon. The putative operon includes eight ORFs which encode proteins homologous to enzymes involved in histidine biosynthesis. The operon also contains (i) an ORF encoding a protein homologous to the histidyl-tRNA synthetases but lacking a motif implicated in synthetase activity, which suggests that it has a role different from tRNA aminoacylation, and (ii) an ORF encoding a protein that is homologous to the 3'-aminoglycoside phosphotransferases but does not confer antibiotic resistance. The remaining ORFs specify products which have no homology with proteins in the EMBL and GenBank data bases.     

Histidine biosynthesis in plants

Summary. The study of histidine metabolism has never been at the forefront of interest in plant systems despite the significant role that the analysis of this pathway has played in development of the field of molecular genetics in microbes. With the advent of methods to analyze plant gene function by complementation of microbial auxotrophic mutants and the complete analysis of plant genome sequences, strides have been made in deciphering the histidine pathway in plants. The studies point to a complex evolutionary origin of genes for histidine biosynthesis. Gene regulation studies have indicated novel regulatory networks involving histidine. In addition, physiological studies have indicated novel functions for histidine in plants as chelators and transporters of metal ions. Recent investigations have revealed intriguing connections of histidine in plant reproduction. The exciting new information suggests that the study of plant histidine biosynthesis has finally begun to flower.     

Toxicity of Copper, Cobalt, and Nickel Salts Is Dependent on Histidine Metabolism in the Yeast Saccharomyces cerevisiae

The pH-dependent inhibition of 22 metal salts have been systematically investigated for the yeast Saccharomyces cerevisiae. We have established that the inhibition of growth by Cu, Co, or Ni salts is markedly enhanced by histidine auxotrophy and by increasing the pH of the medium. Each of the his1-his7 mutant strains were unable to grow in the presence of elevated levels of Cu, Co, or Ni at nearly neutral pHs, in contrast to His(+) strains, which grew under these conditions. The Cu, Co, or Ni inhibition was reversed by the addition of histidine to the medium. Deletion of the high-affinity histidine permease Hip1p in His(-) strains resulted in even greater sensitivity to Cu, Co, and Ni and the requirement of an even higher level of histidine to reverse the inhibition. These results suggest that intracellular histidine, most likely in the vacuole, diminishes the pH-dependent toxicity of Cu, Co, and Ni. Furthermore, the toxicity of many salts is exacerbated in strains with a defective vacuolar H(+)-ATPase, which abolishes the ability of yeast to maintain an acidic vacuole, a compartment known to sequester metal compounds. We suggest that the accumulation of histidine in the vacuole is a normal process used to detoxify Cu, Co, and Ni.     

Derepression and repression of the histidine operon: role of the feedback site of the first enzyme.

Thiazolealanine, a false feedback inhibitor, causes transient repression of the his operon previously derepressed by a severe histidine limitation in strains with a wild-type or feedback-hypersensitive first enzyme but not in feedback-resistant mutants. Since experiments reported here clearly demonstrate that thiazolealanine is not transferred to tRNAHis, it is proposed that this "transient repression" is effected through the interaction of thiazolealanine with the feedback site of the enzyme. Experiments in the presence of rifampin indicate that this thiazolealanine-mediated effect is exerted at the level of translation. We conclude that histidine (free), in addition to forming co-repressor, also represses the operon at the level of translation through feedback interaction with the first enzyme of the pathway (adenosine 5'-triphosphate phosphoribosyltransferase). Rates of derepression in feedback-resistant strains are roughly half of those observed in controls, suggesting a positive role played by a first enzyme with a normal but unoccupied feedback site. Some feedback-resistant mutants, in contrast to the wild type, were unable to exhibit derepression under histidine limitation caused by aminotriazole.     

Histidine operon deattenuation in dnaA mutants of Salmonella typhimurium correlates with a decrease in the gene dosage ratio between tRNA(His) and histidine biosynthetic loci

Expression of the histidine operon of Salmonella typhimurium is increased in dnaA(Ts) mutants at 37 degrees C. This effect requires an intact his attenuator and can be suppressed by increasing the gene copy number of the hisR locus, which encodes the tRNA(His). We present data which suggest that the his deattenuation defect in dnaA(Ts) mutants results from the loss of a gene dosage gradient between the hisR locus, close to oriC, and the his operon, far from oriC. Some of the conclusions drawn here may apply to other operons as well.     

Common Element in the Repression Control of Enzymes of Histidine and Aromatic Amino Acid Biosynthesis in Bacillus subtilus

Single-step mutants of Bacillus subtilis derepressed for enzymes of both aromatic amino acid and histidine biosynthesis were isolated. These mutants occur at a frequency of 10(-6) per cell per generation. All histidine enzymes as well as all enzymes of aromatic acid synthesis which were examined are maximally derepressed. This level cannot be repressed by growth on either histidine or tyrosine. Some of the structural genes which specify the derepressed enzymes are linked to the aromatic cluster; others are unlinked. The significance of these nonrepressible strains is discussed in terms of the mechanism of repression.     

Histidine starvation and completion of the bacterial deoxyribonucleic acid replication cycle

Summary DNA synthesis immediately stops when histidine is removed from exponentially growing cultures of certain histidine-requiring mutants of E. coli and S. typhimurium. Similar observations made elsewhere have led to the proposal that the DNA replication cycle, once initiated, may be under the control of a cycle-completion gene located in or near the histidine operon. However, the present work indicates that in these mutants DNA synthesis is prevented by a shortage of purine nucleotides, and the evidence for a cycle-completion gene can be discounted.     

Histidine starvation and adenosine 5'-triphosphate depletion in chemotaxis of Salmonella typhimurium.

Starvation for histidine prevented tumbling in Salmonella typhimurium hisF auxotrophs, including constantly tumbling strains with an additional mutation in cheB or cheZ. However, histidine-starved cheZs hisF strains were not defective in flagellar function or the tumbling mechanism since freshly starved auxotrophs tumbled in response to a variety of repellents. Tumbling in histidine-starved S. typhimurium could be restored in 13 s by addition of adenine or in 4 min by addition of histidine. Chloramphenicol did not prevent restoration of tumbling by these substances. Assays of adenosine 5'-triphosphate were performed based upon previous demonstration of adenine depletion in hisF auxotrophs starved for histidine. The adenosine 5'-triphosphate concentration dropped rapidly during the course of starvation, falling to less than 5% of the initial level as the cells ceased tumbling entirely. The change to smooth motility was prevented by 2-thiazolealanine, which inhibits phosphoribosyltransferase, thereby preventing adenine depletion during histidine starvation. These results suggest that an adenosine 5'-triphosphate deficiency was responsible for the change in tumbling frequency.