The crystal structure of a bacterial, bifunctional 5,10 methylene-tetrahydrofolate dehydrogenase/cyclohydrolase

The structure of a bifunctional 5,10-methylene-tetrahydrofolate dehydrogenase/cyclohydrolase from Escherichia coli has been determined at 2.5 A resolution in the absence of bound substrates and compared to the NADP-bound structure of the homologous enzyme domains from a trifunctional human synthetase enzyme. Superposition of these structures allows the identification of a highly conserved cluster of basic residues that are appropriately positioned to serve as a binding site for the poly-gamma-glutamyl tail of the tetrahydrofolate substrate. Modeling studies and molecular dynamic simulations of bound methylene-tetrahydrofolate and NADP shows that this binding site would allow interaction of the nicotinamide and pterin rings in the dehydrogenase active site. Comparison of these enzymes also indicates differences between their active sites that might allow the development of inhibitors specific to the bacterial target.

Structural analysis of ternary complexes of Escherichia coli RNA polymerase: ribonuclease footprinting of the nascent RNA in complexes

Ternary complexes of RNA polymerase containing the DNA template and nascent RNA are the intermediates in transcript elongation in all cells. We have footprinted the RNA transcript with single-strand-specific ribonucleases in ternary complexes of Escherichia coli RNA polymerase. When complexes are treated with elevated levels of ribonucleases A and T1, the nascent transcript can be cleaved to within 3-4 nucleotides of the 3'-terminus. Ternary complexes containing ribonuclease-cleaved transcripts as short as 3 nucleotides remain stable and active, ensuring that the cleavage occurred within an active ternary complex. However, cleavage by ribonuclease I is restricted, and gives a limited digest product of about 16 nt. At lower concentrations of ribonuclease T1, two regions of partial protection are seen. The first region extends through the first 15-16 nucleotides from the 3'-OH terminus; the second region extends from position 30 out to position 45. We interpret these regions of partial protection as defining two RNA product binding sites on the RNA polymerase that bind the product to the enzyme during elongation. Our results rule out the existence of a stable RNA-DNA hybrid in these ternary complexes of greater than 3 base pairs in length.

Purification, crystallization, and preliminary X-ray studies of 10-formyltetrahydrofolate synthetase from Clostridia acidici-urici

The monofunctional enzyme 10-formyltetrahydrofolate synthetase (THFS), which is responsible for the recruitment of single carbon units from the formate pool into a variety of folate-dependent biosynthetic pathways, has been subcloned, purified, and crystallized. The crystals belong to space group P2(1), with unit cell dimensions a = 102.4 A, b = 116.5 A, c = 115.8 A, and beta = 103.5. The crystal unit cell and diffraction is consistent with an asymmetric unit consisting of the enzyme tetramer, and a specific volume of the unit cell of 2.7 A3/ Da. The crystals diffract to at least 2.3 A resolution after flash-cooling, when using a rotating anode x-ray source and an RAXIS image plate detector.

Purification, crystallization, and preliminary x-ray studies of a bifunctional 5,10-methenyl/methylene-tetrahydrofolate cyclohydrolase/dehydrogenase from Escherichia coli

A bifunctional enzyme that catalyzes the conversion of formyltetrahydrofolate to methylene-tetrahydrofolate (5,10-methenyltetrahydrofolate cyclohydrolase and 5,10-methylene tetrahydrofolate dehydrogenase), has been subcloned from a cDNA library, purified to homogeneity, and crystallized. The crystals belong to space group I222, with unit cell dimensions of a = 64.5 A, b = 84.9 A, c = 146.1 A. The crystal unit cell and diffraction is consistent with an asymmetric unit consisting of the enzyme monomer, and a specific volume of the unit cell of 3.2 A3/Da. The crystals diffract to at least 2.8 A resolution after flash-cooling, when using a rotating anode x-ray source and an RAXIS image plate detector. A 2.56 A resolution native data set has been collected at beamline X12-C at the NSLS.

Purification, characterization, cloning, and amino acid sequence of the bifunctional enzyme 5,10-methylenetetrahydrofolate dehydrogenase/5,10-methenyltetrahydrofolate cyclohydrolase from Escherichia coli

We have purified the enzyme 5,10-methylenetetrahydrofolate dehydrogenase (EC 1.5.1.5) from Escherichia coli to homogeneity by a newly devised procedure. The enzyme has been purified at least 2,000-fold in a 31% yield. The specific activity of the enzyme obtained is 7.4 times greater than any previous preparation from this source. The purified enzyme is specific for NADP. The protein also contains 5,10-methenyltetrahydrofolate cyclohydrolase (EC 3.5.4.9) activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and behavior on a molecular sieving column suggest that the enzyme is a dimer of identical subunits. We have cloned the E. coli gene coding for the enzyme through the use of polymerase chain reaction based on primers designed from the NH2 terminal analysis of the isolated enzyme. We sequenced the gene. The derived amino acid sequence of the enzyme contains 287 amino acids of Mr 31,000. The sequence shows 50% identity to two bifunctional mitochondrial enzymes specific for NAD, and 40-45% identity to the presumed dehydrogenase/cyclohydrolase domains of the trifunctional C1-tetrahydrofolate synthase of yeast mitochondria and cytoplasm and human and rat cytoplasm. An identical sequence of 14 amino acids with no gaps is present in all 7 sequences.

Positive and negative transcriptional control by heme of genes encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase in Saccharomyces cerevisiae

Responses of the yeast genes encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase, HMG1 and HMG2, to in vivo changes in heme concentrations were investigated. Expression of the genes was determined by direct measurement of the mRNA transcribed from each gene, by direct assay of the enzyme activity encoded by each gene, and by measurement of the expression of lacZ fusions to the control regions of each gene. These studies indicated that expression of HMG1 was stimulated by heme, whereas expression of HMG2 was repressed by heme. The effect of heme on HMG1 expression was mediated by the HAP1 transcriptional regulator and was independent of HAP2. Thus, the genes encoding the 3-hydroxy-3-methylglutaryl coenzyme A reductase isozymes join a growing list of gene pairs that are regulated by heme in opposite ways.

Genetic and molecular characterization of suppressors of SIR4 mutations in Saccharomyces cerevisiae

In order to learn more about other proteins that may be involved in repression of HML and HMR in Saccharomyces cerevisiae, extragenic suppressor mutations were identified that could restore repression in cells defective in SIR4, a gene required for function of the silencer elements flanking HML and HMR. These suppressor mutations, which define at least three new genes, SAN1, SAN2 and SAN3, arose at the frequency expected for loss-of-function mutations following mutagenesis. All san mutations were recessive. Suppression by san1 was allele-nonspecific, since san1 could suppress two very different alleles of SIR4, and was locus-specific since san1 was unable to suppress a SIR3 mutation or a variety of mutations conferring auxotrophies. The SAN1 gene was cloned, sequenced, and used to construct a null allele. The null allele had the same phenotype as the EMS-induced mutations and exhibited no pleiotropies of its own. Thus, the SAN1 gene was not essential. SAN1-mediated suppression was neither due to compensatory mutations in interacting proteins, nor to translational missense suppression. SAN1 may act posttranslationally to control the stability or activity of the SIR4 protein.

Increased amounts of HMG-CoA reductase induce "karmellae": a proliferation of stacked membrane pairs surrounding the yeast nucleus

Overproduction of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in yeast resulted in striking morphological effects on the structure of intracellular membranes. Specifically, stacks of paired membranes closely associated with the nuclear envelope were observed in strains that over-produced the HMG1 isozyme, one of two isozymes for HMG-CoA reductase in yeast. These nuclear-associated, paired membranes have been named "karmellae." In strains that overproduced the HMG1 isozyme, HMG-CoA reductase was present in the karmellar layers. At mitosis, karmellae were asymmetrically segregated: the mother cells inherited all of the karmellae and the daughter cells inherited none. A membranous structure of different morphology was occasionally found in cells that overproduced the HMG2 isozyme. These observations further establish the existence of cellular mechanisms that monitor the levels of membrane proteins and compensate for changes in these levels by inducing synthesis of particular types of membrane.

Enzymatic reactions in the degradation of 5-aminovalerate by Clostridium aminovalericum

The anaerobic degradation of 5-aminovalerate to valerate, acetate, propionate, and ammonia by Clostridium aminovalericum was shown to involve the following intermediates: glutaric semialdehyde, 5-hydroxyvalerate, 5-hydroxyvaleryl-CoA, 4-pentenoyl-CoA, 2,4-pentadienoyl-CoA, trans-2-pentenoyl-CoA, L-3-hydroxyvaleryl-CoA, 3-ketovaleryl-CoA, acetyl- and propionyl-CoA and the corresponding acylphosphates, valeryl-CoA, and possibly 3-pentenoyl-CoA. With exception of the enzyme presumably reducing 2,4-pentadienoyl-CoA to 3-pentenoyl-CoA, enzymes catalyzing the formation and utilization of the above intermediates were demonstrated in extracts. Trans-2-pentenoyl-CoA was shown to be the immediate precursor of valeryl-CoA. The reduction of 2-pentenoyl-CoA was found to be coupled to the oxidation of 4-pentenoyl-CoA to 2,4-pentadienoyl-CoA. Several enzymes catalyzing the above reactions were partially purified and some of their properties determined. A high pressure liquid chromatography method of identifying and estimating most of the above mentioned CoA thiolesters was developed.

p-Cresol formation by cell-free extracts of Clostridium difficile

Cell-free extracts of Clostridium difficile were shown to form p-cresol by decarboxylation of p-hydroxyphenylacetic acid. This activity required both high and low molecular weight fractions. The active component of the low molecular weight fraction had properties of an amino acid and could be replaced by serine, threonine or the corresponding alpha keto acids. Pyruvate was shown to function catalytically. Since the high molecular weight fraction was O2-sensitive and since dithionite was as effective as pyruvate with some high molecular weight fractions, the alpha keto acids probably serve as low potential reducing agents in this system. Because of instability, the p-cresol-forming enzyme could not be purified.