Cloning, sequencing, and expression of a cDNA encoding beta-alanine synthase from rat liver.

A cDNA for beta-alanine synthase from rat liver has been isolated, sequenced, and characterized. beta-Alanine synthase clones were isolated from rat liver cDNA libraries in lambda gt11, using affinity-purified polyclonal antibodies against beta-alanine synthase protein. beta-Alanine synthase protein was not expressed with equal efficiency by all clones. One of the expressed fusion proteins has normal specific enzyme activity, and a second has reduced specific activity. Both clones were completely sequenced and yielded identical DNA sequence, except that one clone contained an additional 36 bases of 5' sequence. The various clones of this cDNA code for an EcoRI insert of 1.5 +/- 0.1 kb, and the open reading frame corresponds to a protein of 393 amino acids (M(r) = 44,042), in good agreement with the M(r) of approximately 42,000 for the native enzyme on SDS-gel electrophoresis. An 11-amino acid sequence was obtained from a tryptic peptide of native beta-alanine synthase; 11 codons for these same amino acids were found at the expected site in the sequenced cDNA, and confirm the open reading frame of the beta-alanine synthase cDNA. Chemical analysis of the native enzyme shows 2 zinc atoms per subunit, and the sequence of beta-alanine synthase contains 2 putative zinc-binding site motifs. Comparison of amino acid sequence, deduced from the cDNA sequence, to sequences in the protein data base showed that it is a unique sequence and that it has about 20% identity to aspartate carbamoyltransferase, ornithine carbamoyltransferase, urease, and leucine aminopeptidase; enzymes that bind comparable ligands or have a similar mechanism.

beta-Alanine synthase: purification and allosteric properties.

beta-Alanine synthase has been purified greater than 1000-fold to homogeneity from rat liver. The enzyme has a subunit molecular weight of 42,000 and a native size of hexamer. The enzyme undergoes ligand-induced changes in polymerization: association in response to the substrate, N-carbamoyl-beta-alanine, and the inhibitor, propionate; and dissociation in response to the product, beta-alanine. The ability of the substrate to associate the pure native enzyme to a larger polymeric species was exploited in the final purification step. The purified enzyme had a pI of 6.7, a Km of 8 microM, and a kcat/Km of 7.9 x 10(4) M-1 s-1. Positive cooperativity was observed toward the substrate N-carbamoyl-beta-alanine, with nH = 1.9. Such cooperativity occurred at substrate concentrations below 12 nM, so that this activation most likely occurs at a regulatory site, with a significantly stronger affinity for N-carbamoyl-beta-alanine than that shown by the catalytic site. The enzyme was sensitive to denaturation, which could be minimized by avoiding heat steps during the purification and by the presence of reducing agents. Such denatured enzyme had little change in Vmax, but had much higher Km, and had also lost the ability to associate or dissociate in response to effectors. After purification, enzyme stability was achieved by the addition of glycerol and detergent.

Structure and regulation of the AMP nucleosidase gene (amn) from Escherichia coli.

The gene for AMP nucleosidase from Escherichia coli (amn) has been sequenced and characterized. The gene codes for a transcript of 1.7 +/- 0.2 kb, and the open reading frame corresponds to a protein of 483 amino acids (Mr = 53848). Amino acid sequences from tryptic peptides of AMP nucleosidase, N-terminal amino acid analysis, and the amino acid composition confirm the gene assignment and the open reading frame of amn. Primer extension studies determined the 5'-end of the amn transcript. The 5'-regulatory region contains overlapping sequences with similarity to the consensus sequences for binding cAMP receptor protein and inorganic phosphate repressor protein. Addition of exogenous cAMP to E. coli deficient in adenylate cyclase resulted in a 3-fold increase in AMP nucleosidase activity. Growth of E. coli on limiting phosphate resulted in an 8-fold increase in the production of AMP nucleosidase. The amn gene was expressed in AMP nucleosidase deficient strains of Azotobacter vinelandii and E. coli. A pUC-amn construct is described that causes approximately 20% of the total protein in E. coli to be produced as AMP nucleosidase. Comparison of the amino acid sequence for AMP nucleosidase with that for yeast AMP deaminase indicates a region in which six of eight amino acids are identical but no other overall homology. The amino acid sequence showed poor agreement with consensus sequences for adenylate binding sites even though the enzyme is known to have a catalytic site for AMP and regulatory sites for MgATP and phosphate.

Characterization of AMD, the AMP deaminase gene in yeast. Production of amd strain, cloning, nucleotide sequence, and properties of the protein.

The structural gene for AMP deaminase (AMD) from Saccharomyces cerevisiae has been cloned and characterized. A yeast strain deficient in AMP deaminase activity was produced and shown to be deficient in AMP deaminase protein by Western blot analysis. The gene for AMP deaminase was located in a lambda gt11 library of yeast genomic DNA, and a DNA fragment from the lambda gt11 clone was used to locate homologous DNA in a yeast genomic library in the centromeric plasmid YCp50, a yeast-Escherichia coli shuttle vector. One plasmid was selected for its ability to restore AMP catalytic activity to the deficient strain. Yeast deficient in AMP deaminase or those overproducing the enzyme grow at near normal rates. The open reading frame corresponding to AMD codes for a protein of 810 amino acids, molecular weight 93,286. The yeast AMD transcript is 3.0 +/- 0.2 kb, and the transcriptional initiation sites have been identified. Western blot analysis of extracts prepared from actively growing yeast indicates a major band at approximately 96,000 molecular weight with several bands at lower molecular weight, including 83,000. When the AMD gene is expressed in E. coli, the large Mr form of AMP deaminase is produced. These results show that the purified enzyme (Mr = 83,000) is a truncated form of the full-length translation product. No adenine nucleotide binding sites were located based on the consensus sequence from other nucleotide binding proteins. No overall homology was found between yeast AMP deaminase and E. coli AMP nucleosidase. Although their metabolic roles and regulatory mechanisms are similar, these enzymes have arisen from separate ancestral proteins.