The pleiotropic effects of his overexpression in Salmonella typhimurium do not involve AICAR-induced mutagenesis

Exploring the role of the histidine biosynthetic hisF gene in cellular metabolism and in the evolution of (ancestral) genes: from LUCA to the extant (micro)organisms

Histidine biosynthesis is an ancestral pathway that was assembled before the appearance of the Last Universal Common Ancestor; afterwards, it remained unaltered in all the extant histidine-synthesizing (micro)organisms. It is a metabolic cross-road interconnecting histidine biosynthesis to nitrogen metabolism and the de novo synthesis of purines. This interconnection is due to the reaction catalyzed by the products of hisH and hisF genes. The latter gene is an excellent model to study which trajectories have been followed by primordial cells to build the first metabolic routes, since its evolution is the result of different molecular rearrangement events, i.e. gene duplication, gene fusion, gene elongation, and domain shuffling. Additionally, this review summarizes data concerning the involvement of hisF and its product in other different cellular processes, revealing that HisF very likely plays a role also in cell division control and involvement in virulence and nodule development in different bacteria. From the metabolic viewpoint, these results suggest that HisF plays a central role in cellular metabolism, highlighting the interconnections of different metabolic pathways.

Biosynthesis of Histidine

The biosynthesis of histidine in Escherichia coli and Salmonella typhimurium has been an important model system for the study of relationships between the flow of intermediates through a biosynthetic pathway and the control of the genes encoding the enzymes that catalyze the steps in a pathway. This article provides a comprehensive review of the histidine biosynthetic pathway and enzymes, including regulation of the flow of intermediates through the pathway and mechanisms that regulate the amounts of the histidine biosynthetic enzymes. In addition, this article reviews the structure and regulation of the histidine (his) biosynthetic operon, including transcript processing, Rho-factor-dependent "classical" polarity, and the current model of his operon attenuation control. Emphasis is placed on areas of recent progress. Notably, most of the enzymes that catalyze histidine biosynthesis have recently been crystallized, and their structures have been determined. Many of the histidine biosynthetic intermediates are unstable, and the histidine biosynthetic enzymes catalyze some chemically unusual reactions. Therefore, these studies have led to considerable mechanistic insight into the pathway itself and have provided deep biochemical understanding of several fundamental processes, such as feedback control, allosteric interactions, and metabolite channeling. Considerable recent progress has also been made on aspects of his operon regulation, including the mechanism of pp(p)Gpp stimulation of his operon transcription, the molecular basis for transcriptional pausing by RNA polymerase, and pathway evolution. The progress in these areas will continue as sophisticated new genomic, metabolomic, proteomic, and structural approaches converge in studies of the histidine biosynthetic pathway and mechanisms of control of his biosynthetic genes in other bacterial species.

Biosynthesis of Histidine

The biosynthesis of histidine in Escherichia coli and Salmonella typhimurium has been an important model system for the study of relationships between the flow of intermediates through a biosynthetic pathway and the control of the genes encoding the enzymes that catalyze the steps in a pathway. This article provides a comprehensive review of the histidine biosynthetic pathway and enzymes, including regulation of the flow of intermediates through the pathway and mechanisms that regulate the amounts of the histidine biosynthetic enzymes. In addition, this article reviews the structure and regulation of the histidine (his) biosynthetic operon, including transcript processing, Rho-factor-dependent "classical" polarity, and the current model of his operon attenuation control. Emphasis is placed on areas of recent progress. Notably, most of the enzymes that catalyze histidine biosynthesis have recently been crystallized, and their structures have been determined. Many of the histidine biosynthetic intermediates are unstable, and the histidine biosynthetic enzymes catalyze some chemically unusual reactions. Therefore, these studies have led to considerable mechanistic insight into the pathway itself and have provided deep biochemical understanding of several fundamental processes, such as feedback control, allosteric interactions, and metabolite channeling. Considerable recent progress has also been made on aspects of his operon regulation, including the mechanism of pp(p)Gpp stimulation of his operon transcription, the molecular basis for transcriptional pausing by RNA polymerase, and pathway evolution. The progress in these areas will continue as sophisticated new genomic, metabolomic, proteomic, and structural approaches converge in studies of the histidine biosynthetic pathway and mechanisms of control of his biosynthetic genes in other bacterial species.

Regulation of hisHF transcription of Aspergillus nidulans by adenine and amino acid limitation

The hisHF gene of Aspergillus nidulans encodes imidazole-glycerole-phosphate (IGP) synthase, consisting of a glutamine amidotransferase and a cyclase domain. The enzyme catalyzes the fifth and sixth steps of histidine biosynthesis, which results in an intermediate of the amino acid and an additional intermediate of purine biosynthesis. An A. nidulans hisHF cDNA complemented a Saccharomyces cerevisiae his7Delta strain and Escherichia coli hisH and hisF mutant strains. The genomic DNA encoding the hisHF gene was cloned and its sequence revealed two introns within the 1659-bp-long open reading frame. The transcription of the hisHF gene of A. nidulans is activated upon amino acid starvation, suggesting that hisHF is a target gene of cross pathway control. Adenine but not histidine, both end products of the biosynthetic pathways connected by the IGP synthase, represses hisHF transcription. In contrast to other organisms HISHF overproduction did not result in any developmental phenotype of the fungus in hyphal growth or the asexual life cycle. hisHF overexpression caused a significantly reduced osmotic tolerance and the inability to undergo the sexual life cycle leading to acleistothecial colonies.

Cell division inhibition in Salmonella typhimurium histidine-constitutive strains: an ftsI-like defect in the presence of wild-type penicillin-binding protein 3 levels

Histidine-constitutive (Hisc) strains of Salmonella typhimurium undergo cell division inhibition in the presence of high concentrations of a metabolizable carbon source. Filaments formed by Hisc strains show constrictions and contain evenly spaced nucleoids, suggesting a defect in septum formation. Inhibitors of penicillin-binding protein 3 (PBP3) induce a filamentation pattern identical to that of Hisc strains. However, the Hisc septation defect is caused neither by reduced PBP3 synthesis nor by reduced PBP3 activity. Gross modifications of peptidoglycan composition are also ruled out. D-Cycloserine, an inhibitor of the soluble pathway producing peptidoglycan precursors, causes phenotypic suppression of filamentation, suggesting that the septation defect of Hisc strains may be caused by scarcity of PBP3 substrate.

Underground metabolism

All enzymes are able to use alternative substrates. When these are naturally occurring metabolites, an 'underground reaction' takes place. Examples are presented in which underground metabolism of this sort produces an observable phenotype. Although biological processes can be remarkably accurate, evolution has selected error rates far from perfect. It is suggested here that a certain level of metabolic inaccuracy, in addition to saving energy, may also confer an evolutionary advantage, for example by providing metabolic plasticity. Since underground reactions are unpredictable from DNA sequence data, caution is in order when interpreting correlations between genetic disorders and pathological syndromes.

Suppression of the pleiotropic effects of HisH and HisF overproduction identifies four novel loci on the Salmonella typhimurium chromosome: osmH, sfiW, sfiX, and sfiY

Insertion mutations that suppress some or all the pleiotropic effects of HisH and HisF overproduction were obtained by using transposons Tn10dTet and Tn10dCam. All suppressor mutations proved to be recessive, indicating that their effects were caused by loss of function; thus, the suppressors identify genes that are necessary to trigger the pleiotropic response when HisH and HisF are overproduced. Genetic mapping of the suppressor mutations identifies four novel loci on the Salmonella typhimurium genetic map. Mutations in osmH (min 49) behave as general suppressors that abolish all manifestations of the pleiotropic response. Mutations in sfiY (min 83) suppress cell division inhibition and thermosensitivity but not osmosensitivity. Mutations that suppress only cell division inhibition define another locus, sfiX (min 44). A fourth novel locus, sfiW (min 19), is also involved in cell division inhibition. The phenotype of sfiW mutations is in turn pleiotropic: they suppress cell division inhibition, make S. typhimurium unable to grow in minimal media, and cause slow growth and abnormal colony and cell shape. The inability of sfiW mutants to grow in minimal medium cannot be relieved by any known nutritional requirement or by the use of carbon sources other than glucose. The hierarchy of suppressor phenotypes and the existence of epistatic effects among suppressor mutations suggest a pathway-like model for the Hisc pleiotropic response.

Excess histidine enzymes cause AICAR-independent filamentation in Escherichia coli

High-level expression of the hisHAFI genes in Escherichia coli, cloned under the control of an IPTG-inducible promoter, caused filamentation, as previously reported in Salmonella typhimurium. We speculated that this filamentation might be produced by an action of the HisH and HisF enzymes on their product AICAR (amino-imidazole carboxamide riboside 5'-phosphate), a histidine by-product and normal purine precursor, possibly by favouring the formation of ZTP, the triphosphate derivative of AICAR. However, filamentation occurred even in the absence of carbon flow through the histidine and purine pathways, as observed in a hisG purF strain lacking the first enzyme in each pathway. Filamentation thus does not require either the normal substrate or products of the overproduced histidine enzymes and must reflect another activity.

AICAR is not an endogenous mutagen in Escherichia coli

A number of observations in the Escherichia coli and Salmonella typhimurium literature could be explained by the hypothesis that a particular purine ribonucleotide precursor can be converted to the corresponding deoxyribonucleotide triphosphate, thereby becoming a base-analogue mutagen. The metabolite in question, AICAR (5-amino-4-carboxamide imidazole riboside 5'-phosphate), is also a by-product of histidine biosynthesis, and its (ribo)triphosphate derivative, ZTP, has been detected in E. coli. We constructed E. coli tester strains that had either a normal AICAR pool (pur+ his+ strains cultivated without purines or histidine) or no AICAR pool (purF hisG mutant strains, lacking the first enzyme of each pathway and cultivated in the presence of adenine and histidine). Using a set of lacZ mutations, each of which can revert to Lac+ only by a specific substitution mutation, we found that no base substitution event occurs at a higher frequency in the presence of an AICAR pool. We conclude that the normal AICAR pool in E. coli is not a significant source of spontaneous base substitution mutagenesis.