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At least 19 recordsLinked to original sources
PubMed1987

Kinetic mechanism of histidinol dehydrogenase: histidinol binding and exchange reactions.

Salmonella typhimurium histidinol dehydrogenase produces histidine from the amino alcohol histidinol by two sequential NAD-linked oxidations which form and oxidize a stable enzyme-bound histidinaldehyde intermediate. The enzyme was found to catalyze the exchange of 3H between histidinol and [4(R)-3H]NADH and between NAD and [4(S)-3H]NADH. The latter reaction proceeded at rates greater than kcat for the net reaction and was about 3-fold faster than the former. Histidine did not support an NAD/NADH exchange, demonstrating kinetic irreversibility in the second half-reaction. Specific activity measurements on [3H]histidinol produced during the histidinol/NADH exchange reaction showed that only a single hydrogen was exchanged between the two reactants, demonstrating that under the conditions employed this exchange reaction arises only from the reversal of the alcohol dehydrogenase step and not the aldehyde dehydrogenase reaction. The kinetics of the NAD/NADH exchange reaction demonstrated a hyperbolic dependence on the concentration of NAD and NADH when the two were present in a 1:2 molar ratio. The histidinol/NADH exchange showed severe inhibition by high NAD and NADH under the same conditions, indicating that histidinol cannot dissociate directly from the ternary enzyme-NAD-histidinol complex; in other words, the binding of substrate is ordered with histidinol leading. Binding studies indicated that [3H]histidinol bound to 1.7 sites on the dimeric enzyme (0.85 site/monomer) with a KD of 10 microM. No binding of [3H]NAD or [3H]NADH was detected. The nucleotides could, however, displace histidinol dehydrogenase from Cibacron Blue-agarose.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcohol Oxidoreductases↗
PubMed2001

Crystal structure of histidinol phosphate aminotransferase (HisC) from Escherichia coli, and its covalent complex with pyridoxal-5'-phosphate and l-histidinol phosphate.

The biosynthesis of histidine is a central metabolic process in organisms ranging from bacteria to yeast and plants. The seventh step in the synthesis of histidine within eubacteria is carried out by a pyridoxal-5'-phosphate (PLP)-dependent l-histidinol phosphate aminotransferase (HisC, EC 2.6.1.9). Here, we report the crystal structure of l-histidinol phosphate aminotransferase from Escherichia coli, as a complex with pyridoxamine-5'-phosphate (PMP) at 1.5 A resolution, as the internal aldimine with PLP, and in a covalent, tetrahedral complex consisting of PLP and l-histidinol phosphate attached to Lys214, both at 2.2 A resolution. This covalent complex resembles, in structural terms, the gem-diamine intermediate that is formed transiently during conversion of the internal to external aldimine.HisC is a dimeric enzyme with a mass of approximately 80 kDa. Like most PLP-dependent enzymes, each HisC monomer consists of two domains, a larger PLP-binding domain having an alpha/beta/alpha topology, and a smaller domain. An N-terminal arm contributes to the dimerization of the two monomers. The PLP-binding domain of HisC shows weak sequence similarity, but significant structural similarity with the PLP-binding domains of a number of PLP-dependent enzymes. Residues that interact with the PLP cofactor, including Tyr55, Asn157, Asp184, Tyr187, Ser213, Lys214 and Arg222, are conserved in the family of aspartate, tyrosine and histidinol phosphate aminotransferases. The imidazole ring of l-histidinol phosphate is bound, in part, through a hydrogen bond with Tyr110, a residue that is substituted by Phe in the broad substrate specific HisC enzymes from Zymomonas mobilis and Bacillus subtilis. Comparison of the structures of the HisC internal aldimine, the PMP complex and the HisC l-histidinol phosphate complex reveal minimal changes in protein or ligand structure. Proton transfer, required for conversion of the gem-diamine to the external aldimine, does not appear to be limited by the distance between substrate and lysine amino groups. We propose that the tetrahedral complex has resulted from non-productive binding of l-histidinol phosphate soaked into the HisC crystals, resulting in its inability to be converted to the external aldimine at the HisC active site.

Amino Acid Sequence↗
PubMed1986

A cysteine residue (cysteine-116) in the histidinol binding site of histidinol dehydrogenase.

Salmonella typhimurium L-histidinol dehydrogenase (EC 1.1.1.23), a four-electron dehydrogenase, was inactivated by an active-site-directed modification reagent, 7-chloro-4-nitro-2,1,3-benzoxadiazole (NBD-Cl). The inactivation followed pseudo-first-order kinetics and was prevented by low concentrations of the substrate L-histidinol or by the competitive inhibitors histamine and imidazole. The observed rate saturation kinetics for inactivation suggest that NBD-Cl binds to the enzyme noncovalently before covalent inactivation occurs. The UV spectrum of the inactivated enzyme showed a peak at 420 nm, indicative of sulfhydryl modification. Stoichiometry experiments indicated that full inactivation was correlated with modification of 1.5 sulfhydryl groups per subunit of enzyme. By use of a substrate protection scheme, it was shown that 0.5 sulfhydryl per enzyme subunit was neither protected against NBD-Cl modification by L-histidinol nor essential for activity. Modification of the additional 1.0 sulfhydryl caused complete loss of enzyme activity and was prevented by L-histidinol. Pepsin digestion of NBD-modified enzyme was used to prepare labeled peptides under conditions that prevented migration of the NBD group. HPLC purification of the peptides was monitored at 420 nm, which is highly selective for NBD-labeled cysteine residues. By amino acid sequencing of the major peptides, it was shown that the reagent modified primarily Cys-116 and Cys-377 and that the presence of L-histidinol gave significant protection of Cys-116. The presence of a cysteine residue in the histidinol binding site is consistent with models in which formation and subsequent oxidation of a thiohemiacetal occurs as an intermediate step in the overall reaction.

4-Chloro-7-nitrobenzofurazan↗
PubMed2001

Structures of Escherichia coli histidinol-phosphate aminotransferase and its complexes with histidinol-phosphate and N-(5'-phosphopyridoxyl)-L-glutamate: double substrate recognition of the enzyme.

Histidinol-phosphate aminotransferase (HspAT) is a key enzyme on the histidine biosynthetic pathway. HspAT catalyzes the transfer of the amino group of L-histidinol phosphate (Hsp) to 2-oxoglutarate to form imidazole acetol phosphate (IAP) and glutamate. Thus, HspAT recognizes two kinds of substrates, Hsp and glutamate (double substrate recognition). The crystal structures of native HspAT and its complexes with Hsp and N-(5'-phosphopyridoxyl)-L-glutamate have been solved and refined to R-factors of 19.7, 19.1, and 17.8% at 2.0, 2.2, and 2.3 A resolution, respectively. The enzyme is a homodimer, and the polypeptide chain of the subunit is folded into one arm, one small domain, and one large domain. Aspartate aminotransferases (AspATs) from many species were classified into aminotransferase subgroups Ia and Ib. The primary sequence of HspAT is less than 18% identical to those of Escherichia coli AspAT of subgroup Ia and Thermus thermophilus HB8 AspAT of subgroup Ib. The X-ray analysis of HspAT showed that the overall structure is significantly similar to that of AspAT of subgroup Ib rather than subgroup Ia, and the N-terminal region moves close to the active site like that of subgroup Ib AspAT upon binding of Hsp. The folding of the main-chain atoms in the active site is conserved between HspAT and the AspATs, and more than 40% of the active-site residues is also conserved. The eHspAT recognizes both Hsp and glutamate by utilizing essentially the same active-site folding as that of AspAT, conserving the essential residues for transamination reaction, and replacing and relocating some of the active-site residues. The binding sites for the phosphate and the alpha-carboxylate groups of the substrates are roughly located at the same position and those for the imidazole and gamma-carboxylate groups at the different positions. The mechanism for the double substrate recognition observed in eHspAT is in contrast to that in aromatic amino acid aminotransferase, where the recognition site for the side chain of the acidic amino acid is formed at the same position as that for the side chain of aromatic amino acids by large-scale rearrangements of the hydrogen bond networks.

Amino Acid Sequence↗
PubMed1988

Effect of L-histidinol on the metabolism of 5-fluorouracil in the BALB/c x DBA/8 F1 murine tumor system.

L-Histidinol, a structural analogue of histidine, which transiently inhibits proliferation, can protect cells from the toxic effects of proliferation-dependent chemotherapeutic agents such as 5-fluorouracil (FUra). In the BALB/c x DBA/8 F1 (hereafter called CD8F1) murine tumor system, L-histidinol protected mice from FUra-induced leukopenia, weight loss, and mortality; however, the therapeutic index was not improved since L-histidinol also protected the tumor against the toxic effects of FUra. In order to understand the mechanism of this protection, we examined the effects of L-histidinol on the metabolism of FUra. Results indicate that L-histidinol had no effect on the phosphoribosyl pyrophosphate levels in tumor, the activation of FUra to nucleotides or levels of free 5-fluorodeoxyuridine monophosphate in either tumor or bone marrow. L-Histidinol (7 mg/mouse, every 2 h for 5 doses) reduced RNA and DNA synthesis, as measured by 32P incorporation in vivo, by approximately one-half in tumor, and by 70% in bone marrow. This in turn resulted in reduced incorporation of FUra into RNA in both tumor and bone marrow. At 2 h, 4 h, and 24 h after FUra administration the level of FUra in RNA was 24-37% less in both tumor and bone marrow of mice that received L-histidinol with FUra. Using 32P as a monitor of overall RNA synthesis, the [3H]FUra/32P ratio remained unchanged, suggesting that the reduction of FUra incorporation into RNA was due to decreased RNA synthesis rather than a decrease in the number of FUra molecules per RNA chain. In contrast, L-histidinol had no effect on the in vivo inhibition of thymidylate synthetase by 5-fluorodeoxyuridine monophosphate as measured by the incorporation of [3H]-2'-deoxyuridine into DNA or on the percentages of thymidylate synthetase in the free versus 5-fluorodeoxyuridine monophosphate-bound state. We conclude that L-histidinol reduces FUra toxicity by reducing FUra incorporation into RNA. Since the major mechanism of action in the CD8F1 breast tumor system is the incorporation of FUra into RNA, the reduction in toxicity and antitumor activity observed when L-histidinol is combined with FUra is consistent with the observed reduction in tumor and bone marrow RNA containing incorporated FUra residues.

Animals↗
PubMed1984

'Stress-proteins' are induced in Tetrahymena pyriformis by histidinol but not in mammalian (L-929) cells.

This study compares the influence of histidinol, a reversible inhibitor of growth and protein synthesis, on patterns of proteins synthesis in Tetrahymena pyriformis and cultured mouse L cells. In Tetrahymena, histidinol (10 mM) reduced total cell protein synthesis by over 60%. Analysis of individual protein bands by SDS-PAGE showed that at least 17 bands became less prominent, while 13 polypeptides became more prominent, during exposure to histidinol. These effects were rapidly reversed by addition of equivalent concentrations of histidine. Actinomycin D (actD) prevented the enhancement of most of the histidinol-'stimulated' bands, suggesting control at the transcriptional level. The majority of the histidinol-'stimulated' polypeptides appeared to be the same as corresponding heat-shock polypeptides, although some polypeptides were 'stimulated' by histidinol and not by heat shock, while others were 'stimulated' by heat shock and not by histidinol. 'Stimulation', both by histidinol and by heat shock, may reflect selective retention rather than induced synthesis of some or all of the relevant polypeptides. In contrast to Tetrahymena, cultured mouse L cells did not alter their normal polypeptide pattern under the influence of histidinol, under conditions in which total protein synthesis was depressed to a similar degree. It is possible that 'stress' proteins might be formed or retained in order to mediate adaptive responses of free-living cells to environmental changes.

Animals↗
PubMed1989

Infused L-histidinol and cisplatin: schedule, specificity, and proliferation dependence.

The dose and schedule requirements found for the combination of L-histidinol and 5-fluorouracil (5-FU) were concordant with those for the combination of L-histidinol and cisplatin. Furthermore, cisplatin-L-histidinol was active against colon 26 tumor, an adenocarcinoma that developed in a BALB/c female mouse and that has been grown as a solid tumor. The toxicity of cisplatin was prevented only when cisplatin was given before L-histidinol. Studies of L-histidinol and 5-FU had similar results. For (DBA/2 X BALB/c)F1 mice, 50 mg of L-histidinol per mouse was required for protection; for hematopoietic precursor cells, protection was dependent on the dose of L-histidinol. In contrast, both L1210 leukemia cells and colon 26 adenocarcinoma cells were more efficiently killed by combinations of L-histidinol and cisplatin. This effect depended on the doses of L-histidinol and cisplatin, a finding similar to the finding for hematopoietic precursor cells.

Adenocarcinoma↗
PubMedDate not supplied

A novel approach for improving the efficacy of experimental cancer chemotherapy using combinations of anticancer drugs and L-histidinol.

One of the major limitations to the chemical management of human malignancies is the failure of most antineoplastic agents to act specifically against tumour cells. A novel approach for improving both the specificity and the efficacy of experimental cancer chemotherapy is described in this review. The approach is based upon the use of L-histidinol in combination with conventional anticancer drugs. L-Histidinol, a structural analogue of the essential amino acid L-histidine, is a reversible inhibitor of protein biosynthesis which evokes disparate responses from non-tumorigenic and tumorigenic cells in culture. Whereas L-histidinol protects a wide variety of phenotypically normal cells from anticancer drug toxicity, it enhances the vulnerability of tumorigenic cells to the same agents. More importantly, these remarkable properties of L-histidinol are retained in tumour-bearing animals. Thus, L-histidinol diminishes the myelocytoxicity otherwise associated with the in vivo use of agents such as cytosine arabinoside and 5-fluorouracil. Simultaneously, L-histidinol increases the inherent capacities of these two antimetabolites to eradicate in situ tumour cells. More recently, it has been found that L-histidinol can increase both the specificity and the efficacy of a number of other antineoplastic agents. For example, alkylating agents such as BCNU, cyclophosphamide and cis-platinum, as well as the antitumour antibiotic daunomycin, can be combined with L-histidinol to provide curative treatment for tumour-bearing animals under conditions where these drugs, on their own, have little or no impact on survival. These results demonstrate that the L-histidinol/anticancer drug combination approach to chemotherapy is effective with a variety of clinically-relevant antineoplastic agents. However, it remains to be demonstrated whether this approach will prove applicable in, or effective for, human cancer chemotherapy.

Animals↗
PubMed1999

Mechanism of Salmonella typhimurium histidinol dehydrogenase: kinetic isotope effects and pH profiles.

L-Histidinol dehydrogenase catalyzes the biosynthetic oxidation of L-histidinol to L-histidine with sequential reduction of two molecules of NAD. Previous isotope exchange results had suggested that the oxidation of histidinol to the intermediate histidinaldehyde occurred 2-3-fold more rapidly than overall catalysis. In this work, we present kinetic isotope effects (KIE) studies at pH 9.0 and at pH 6.7 with stereospecifically mono- and dideuterated histidinols. The data at pH 9.0 support minimal participation of the first hydride transfer and substantial participation of the second hydride transfer in the overall rate limitation. Stopped-flow experiments with protiated histidinol revealed a small burst of NADH production with stoichiometry of 0.12 per subunit, and 0.25 per subunit with dideuterated histidinol, indicating that the overall first half-reaction was not significantly faster than the second reaction sequence. Results from kcat and kcat/KM titrations with histidinol, NAD, and the alternative substrate imidazolyl propanediol demonstrated an essential base with pKa values between 7.7 and 8.4. In KIE experiments performed at pH 6.7 or with a coenzyme analogue at pH 9. 0, the first hydride transfer became more rate limiting. Kinetic simulations based on rate constants estimated from this work fit well with a mechanism that includes a relatively fast, and thermodynamically unfavorable, hydride transfer from histidinol and a slower, irreversible second hydride transfer from a histidinaldehyde derivative. Thus, although the chemistry of the first hydride transfer is fast, both partial reactions participate in the overall rate limitation.

Alcohol Oxidoreductases↗
PubMed1994

Steady-state kinetics of cabbage histidinol dehydrogenase.

Cabbage histidinol dehydrogenase (HDH) oxidizes L-histidinol to L-histidine through two sequential NAD(+)-linked reactions via an alkaline-labile, L-histidinaldehyde intermediate. The kinetic mechanism of the overall reaction as well as the partial reactions involved in the overall catalysis were investigated at pH 7.2 using L-histidinaldehyde as a substrate. Product inhibition patterns conformed to a Bi Uni Uni Bi Ping Pong mechanism as reported for the HDH from Salmonella typhimurium. Thus, the reaction scheme is ordered with the binding of histidinol first and NAD+ second, and histidine is the last product to be released. The intermediate, L-histidinaldehyde, could be a substrate for both the oxidation and the reduction reactions to produce histidine and histidinol, respectively. L-Histidine was not enzymatically reduced in the presence of NADH, indicating that the reaction to oxidize histidinaldehyde is apparently irreversible. L-Histidinaldehyde exhibited a three times greater binding rate constant than histidinol with a considerably small dissociation constant. These results were in agreement with the observation that histidinaldehyde was not released during the overall reaction. The rate of the reduction of histidinaldehyde to histidinol was almost same as that of the overall oxidation reaction. The overall oxidation from histidinol to histidine proceeded about three times slower than the partial oxidation from histidinaldehyde to histidine, suggesting that the first-half forward reaction is the rate-determining step in the total reaction of cabbage HDH.

Alcohol Oxidoreductases↗
PubMed1996

Transgenic mice for the establishment of histidinol-resistant embryonic fibroblast feeder layers.

Gene targeting in mouse embryonic stem cells generates mutations by replacing an endogenous chromosomal region with a copy disrupted by a selectable genetic marker. The most commonly used selectable marker is the bacterial neo(r) gene, which confers resistance in mammalian cells to the antibiotic G418. Use of an alternative selectable marker, the Salmonella typhimurium gene hisD, should provide expanded applications for gene targeting. The hisD gene encodes the protein histidinol dehydrogenase, which catalyzes the conversion of histidinol to the amino acid histidine. Histidinol is toxic to mammalian cells, while histidine is an essential mammalian amino acid. Consequently, growth selection in cultures with media containing histidinol in place of histidine occurs by both histidine starvation and histidinol poisoning. The hisD selection is being tested for potential use in gene targeting experiments with mouse embryonic stem (ES) cells. Currently, most successful gene targeting experiments use primary embryonic fibroblast feeder layers, which assist in the maintenance of the pluripotential state of the embryonic stem cells. To support ES cell stability under histidinol selection, mice transgenic for the S. typhimurium hisD gene have been produced and used to generate embryonic fibroblast feeder cells. The transgenic embryonic fibroblasts survive under a wide range of histidinol-containing growth conditions and support growth of ES cell cultures.

Alcohol Oxidoreductases↗
PubMed1987

Effects of L-histidinol on the susceptibility of P815 mastocytoma cells to selected anticancer drugs in vitro and in DBA/2J mice.

The effects of L-histidinol on the susceptibility of the transplantable murine mast-cell neoplasm P815 mastocytoma to selected anticancer drugs have been evaluated on cells growing in culture and in syngeneic DBA/2J mice. Combinations of L-histidinol and anticancer drugs of either phase specificity [cytarabine (ara-C) and vinblastine sulfate] or cycle specificity [5-fluorouracil (FUra) and methotrexate] had diverse effects on cultured mastocytoma cells as scored by clonogenic cell survival assays. Flow cytometric analysis of randomly proliferating P815 mastocytoma cells revealed that although exposure to L-histidinol did not preclude cells from traversing the cell cycle, the analogue nonetheless conferred a dose-dependent and apparently nonspecific delay of cell cycle transit. DBA/2J mice bearing intraperitoneal P815 mastocytoma cells were used to evaluate the in vivo efficacy of L-histidinol-ara-C and of L-histidinol-FUra combinations. Quantitative cell survival assays of murine bone marrow cells and of clonogenic tumor cells obtained from treated animals demonstrated that L-histidinol eliminated the bone marrow toxicity otherwise attending the use of the drugs ara-C and FUra. Simultaneously, the inclusion of L-histidinol provided a statistically significant increase in the capacity of these two anticancer drugs to eradicate intraperitoneal mastocytoma cells.

Animals↗
PubMed1987

Failure of L-histidinol to improve the therapeutic efficiency of 5-fluorouracil against murine breast tumors.

It has been reported that L-histidinol, a structural analogue of the essential amino acid L-histidine, can transiently inhibit proliferative cycling in cells with normal phenotype while allowing continued cell cycle transit in tumor cells. Thus, in the presence of L-histidinol, the toxicity of a proliferation-dependent drug such as 5-fluorouracil (FUra) was found to be reduced in normal tissue cells of the DBA/2J mouse, but not in L1210 leukemia cells in the same mouse. Because of the potential clinical significance of this approach to reduce chemotherapy-associated host toxicity, we evaluated the L-histidinol-FUra combination in a nonleukemic, solid murine tumor model, the BALB/c X DBA/8 F1 (hereafter called CD8F1) breast tumor. The results of these studies indicate that the administration of L-histidinol can protect the CD8F1 mouse from FUra-associated leukopenia, body weight loss, and ultimately, from mortality. However, in contrast to results reported in the L1210 leukemic system, L-histidinol also reduced the cytotoxic activity of FUra against CD8F1 breast tumors. Therefore, although the dose of FUra that could be administered with safety was higher in mice receiving L-histidinol, the therapeutic results of the combination of FUra and L-histidinol were not superior to those obtained with FUra alone at a lower dose.

Animals↗
PubMed2004

Structural studies of the catalytic reaction pathway of a hyperthermophilic histidinol-phosphate aminotransferase.

In histidine biosynthesis, histidinol-phosphate aminotransferase catalyzes the transfer of the amino group from glutamate to imidazole acetol-phosphate producing 2-oxoglutarate and histidinol phosphate. In some organisms such as the hyperthermophile Thermotoga maritima, specific tyrosine and aromatic amino acid transaminases have not been identified to date, suggesting an additional role for histidinol-phosphate aminotransferase in other transamination reactions generating aromatic amino acids. To gain insight into the specific function of this transaminase, we have determined its crystal structure in the absence of any ligand except phosphate, in the presence of covalently bound pyridoxal 5'-phosphate, of the coenzyme histidinol phosphate adduct, and of pyridoxamine 5'-phosphate. The enzyme accepts histidinol phosphate, tyrosine, tryptophan, and phenylalanine, but not histidine, as substrates. The structures provide a model of how these different substrates could be accommodated by histidinol-phosphate aminotransferase. Some of the structural features of the enzyme are more preserved between the T. maritima enzyme and a related threonine-phosphate decarboxylase from S. typhimurium than with histidinol-phosphate aminotransferases from different organisms.

Amino Acid Sequence↗
PubMed1989

L-histidinol improves the selectivity and efficacy of alkylating agents and daunomycin in mice with P388 leukaemia.

DBA/2J mice bearing a clonal isolate of the transplantable murine lymphocytic leukaemia line P388 were used to examine the effects of L-histidinol on the antitumour activity of three alkalyating agents (bis-chloroethylnitrosourea (BCNU), cis-diamminedichloroplatinum (II) (cisDDP) and cyclophosphamide) and the antitumour antibiotic daunomycin. Single, combined treatments with L-histidinol and either BCNU or cisDDP, at doses of the alkylating agents which were ineffective when used alone, were completely curative. Dose-response studies showed that L-histidinol conferred dose-dependent, synergistic improvements on the capacities of both BCNU and cisDDP to increase the life-span of DBA/2J mice bearing P388 leukemia. For combinations of L-histidinol and cyclophosphamide or daunomycin, two successive treatments with L-histidinol and drug were required to obtain a significant portion of long-term survivors. Thus, in this model system, the L-histidinol/anticancer drug combination approach for improving experimental cancer chemotherapy can be employed successfully with three alkylating agents and the antitumour antibiotic daunomycin.

Alkylating Agents↗
PubMed1975

Regulation of macromolecular synthesis in reovirus-infected L-929 cells I. Effect of L-histidinol.

The histidine analogue L-histidinol, reported by Vaughan and Hansen (1973) to establish a potent, readily reversible inhibition of eukaryotic protein synthesis in vivo, was used to investigate the regulation of macromolecular synthesis in reovirus-infected L-929 cells. The addition of L-histidinol to normal L cells led to a total inhibition of protein synthesis. The inhibition appeared to be a consequence neither of isotope dilution resulting from elevated endogenous amino acids nor of an inability of treated cells to accumulate exogenous amino acids. Addition of L-histidine to histidinol-arrested cells resulted in a complete recovery of protein synthesis. Similarly, protein synthesis in reovirus-infected L cells examined 17 h postinfection (31 C) was totally inhibited by histidinol treatment and was readily reversed by the addition of histidine. Reovirus-infected cells treated with histidinol had an essentially unaltered capacity to synthesize reovirus single-stranded RNA relative to unperturbed cultures but a diminishing ability to maintain genome RNA synthesis. Addition of L-histidine to arrested cultures led to a complete recovery of genome RNA synthesis. The L-histidinol-mediated arrest of protein synthesis was both very effective and easily reversed, suggesting the general applicability of this novel inhibitor to investigations of regulation of macromolecular synthesis in both normal and virus-infected eukaryotic cells.

Histidine↗