Search PubMed⌕ Search

Biomedical subjects

H Tabor

Publications and source records attributed to H Tabor.

At least 37 records · Page 2Linked to original sources
PubMed1989

Spermidine biosynthesis in Escherichia coli: promoter and termination regions of the speED operon.

Two enzymes, S-adenosylmethionine decarboxylase and spermidine synthase, are essential for the biosynthesis of spermidine in Escherichia coli. We have previously shown that the genes encoding these enzymes (speD and speE) form an operon and that the area immediately upstream from the speE gene is necessary for the expression of both the speE and speD genes. We have now studied the upstream promoter and the downstream terminator regions of this operon more completely. We have shown that the major mRNA initiation site (Ia) of the operon is located 475 base pairs (bp) upstream from the speE gene and that there is an open reading frame that encodes for a polypeptide of 115 amino acids between the Ia site and the ATG start codon for the speE gene. Downstream from the stop codon for the speD gene is a potential hairpin structure immediately followed by an mRNA termination site, t. An additional mRNA termination site, t', is present about 110 bp downstream from t and is stronger than t. By comparing our DNA fragments with those prepared from this region of the E. coli chromosome by Kohara et al., we have located the speED operon on the physical map of the E. coli chromosome. We have shown that the orientation of the speED operon is counterclockwise and that the operon is located 137.5 to 140 kbp (2.9 minutes) clockwise from the zero position of the E. coli chromosomal map.

Adenosylmethionine Decarboxylase↗
PubMed1987

The speEspeD operon of Escherichia coli. Formation and processing of a proenzyme form of S-adenosylmethionine decarboxylase.

We have previously shown that the gene (speD) for S-adenosylmethionine decarboxylase is part of an operon that also contains the gene (speE) for spermidine synthase (Tabor, C. W., Tabor, H., and Xie, Q.-W. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 6040-6044). We have now determined the nucleotide sequence of this operon and have found that speD codes for a polypeptide of Mr = 30,400, which is considerably greater than the subunit size of the purified enzyme. Our studies show that S-adenosylmethionine decarboxylase is first formed as a Mr = 30,400 polypeptide and that this proenzyme is then cleaved at the Lys111-Ser112 peptide bond to form a Mr = 12,400 subunit and a Mr = 18,000 subunit. The latter subunit contains the pyruvoyl moiety that we previously showed is required for enzymatic activity. Both subunits are present in the purified enzyme. These conclusions are based on (i) pulse-chase experiments with a strain containing a speD+ plasmid which showed a precursor-product relationship between the proenzyme and the enzyme subunits, (ii) the amino acid sequence of the proenzyme form of S-adenosylmethionine decarboxylase (derived from the nucleotide sequence of the speD gene), and (iii) comparison of this sequence of the proenzyme with the N-terminal amino acid sequences of the two subunits of the purified enzyme reported by Anton and Kutny (Anton, D. L., and Kutny, R. (1987) J. Biol. Chem. 262, 2817-2822).

Adenosylmethionine Decarboxylase↗
PubMed1986

Spermidine synthase of Escherichia coli: localization of the speE gene.

We have obtained Escherichia coli mutants lacking spermidine synthase (putrescine aminopropyltransferase) and have found that the mutated gene (speE) is located immediately upstream from the gene coding for S-adenosylmethionine decarboxylase (speD); these genes are located at 2.7 minutes on the E. coli chromosome. Both genes are present in a 1795-base-pair fragment of E. coli DNA that was cloned into pBR322. Deletion of 105 bases upstream of speE caused a coordinate loss of both activities, indicating that speE and speD constitute a single operon. speE and speD have also been cloned separately in a high-expression vector; strains carrying these plasmids overproduce the respective enzymes.

Adenosylmethionine Decarboxylase↗
PubMed1984

Expression of the cloned genes encoding the putrescine biosynthetic enzymes and methionine adenosyltransferase of Escherichia coli (speA, speB, speC and metK).

The speA, speB and speC genes, which code for arginine decarboxylase (ADCase), agmatine ureohydrolase (AUHase) and ornithine decarboxylase (ODCase), respectively, and the metK gene, which encodes methionine adenosyltransferase (MATase), have been cloned. The genes were isolated from hybrid ColE1 plasmids of the Clarke-Carbon collection and were ligated into plasmid pBR322. Escherichia coli strains transformed with the recombinant plasmids exhibit a 7- to 17-fold overproduction of the various enzymes, as estimated from increases in the specific activities of the enzymes assayed in crude extracts. Minicells bearing the pBR322 hybrid plasmids and labeled with radioactive lysine synthesize radiolabeled proteins with Mrs corresponding to those reported for purified ODCase, ADCase and MATase. Restriction enzyme analysis of the plasmids, combined with measurements of specific activities of the enzymes in crude extracts of cells bearing recombinant plasmids, clarified the relative position of speA and speB. The gene order in the 62- to 64-min region is serA speB speA metK speC glc.

Carboxy-Lyases↗
PubMed1984

Specificity of polyamine requirements for the replication and maintenance of different double-stranded RNA plasmids in Saccharomyces cerevisiae.

We have shown previously that the M1 double-stranded (ds) RNA (i.e., the killer plasmid [KIL-k1]) that codes for a protein toxin requires spermidine or spermine for its replication. We now report that replication of two other ds RNA plasmids of yeast also requires polyamines: (i) M2 ds RNA [( KIL-k2]) and (ii) L-A-E, a ds RNA plasmid carrying the non-Mendelian genetic element [EXL]. Putrescine alone is sufficient to maintain L-A-E but is not sufficient to maintain either M1 ds RNA or M2 ds RNA, which require either spermidine or spermine. Once M1 or M2 or L-A-E is lost, it cannot be restored by the addition of polyamines. In contrast, L-A-HN, a ds RNA molecule that carries the cytoplasmic genes [HOK] and [NEX], is not lost during polyamine deprivation. It is striking that polyamine deprivation differentially affects L-A-E and L-A-HN, even though these two ds RNA molecules have more than 99% homology. L-C, which is the same size as L-A but very different in sequence, is also not lost on polyamine starvation.

Genotype↗
PubMed1984

Polyamines.

Explore the source record for details and available documents.

Adenosylmethionine Decarboxylase↗