The pAR5 mutation and the allosteric mechanism of Escherichia coli aspartate carbamoyltransferase.

Mutation pAR5 replaces residues 145'-153' at the C terminus of the regulatory (r) chains of Escherichia coli ATCase by a new sequence of six residues. The mutated enzyme has been shown to lack substrate cooperativity and inhibition by CTP. Solution X-ray scattering curves demonstrate that, in the absence of ligands, its structure is intermediate between the T form and the R form. In the presence of N-phosphonacetyl-L-aspartate, the mutant is similar to the wild type. An examination of the crystal structure of unligated ATCase reveals that the mutated site is at an interface between r and catalytic (c) chains, which exists only in the T allosteric form. A computer simulation by energy minimization suggests that the pAR5 mutation destabilizes this interface and induces minor changes in the tertiary structure of r chains. The resulting lower stability of the T form explains the loss of substrate cooperativity. The lack of allosteric inhibition may be related to a new electrostatic interaction made in mutant r chains between the C-terminal carboxylate and a lysine residue of the allosteric domain.     

Import of alcohol oxidase into peroxisomes of Saccharomyces cerevisiae.

Saccharomyces cerevisiae is unable to grow on methanol because it lacks the enzymes required for its metabolism. To study the possibility of whether or not the methanol oxidation pathway of Hansenula polymorpha can be transferred to S. cerevisiae, the gene coding for alcohol oxidase, a peroxisomal homo-octameric flavoprotein, was introduced into S. cerevisiae. Transformed cells contain varying amounts of alcohol oxidase depending on the plasmid used. Immunocytochemical experiments indicate that the protein is imported into peroxisomes, whether organelle proliferation is induced or not. Cells lack alcohol oxidase activity however, and only the monomeric, non-functional, form of the protein is found. These findings indicate that the H. polymorpha peroxisomal targeting signal of alcohol oxidase is recognized in S. cerevisiae and protein monomers are imported.     

Construction of hybrid Tn501/Tn21 transposases in vivo: identification of a region of transposase conferring specificity of recognition of the 38-bp terminal inverted repeats.

In order to study the transposase enzymes of Class II prokaryotic transposable elements, we have constructed genes encoding hybrid transposase proteins. This was done by recombination in vivo between the tnpA genes of transposons Tn501 and Tn21. These hybrid genes can complement in trans a transposition-defective mutant of Tn501. The structures of the products of this complementation indicate whether the specificity of the hybrid transposase in recognising the 38 bp terminal inverted repeats is that of Tn501 or that of Tn21. The determinant of this specificity is in the N-terminal region of the transposase protein, between amino acids 28 and 216. The predicted amino acid sequences so far determined of transposases from the Class II family reveal an area of homology in this region.     

Cloning and expression of the gene for a fibronectin-binding protein from Staphylococcus aureus.

The gene encoding the fibronectin-binding protein (FNBP) from Staphylococcus aureus strain 8325-4 was isolated from a gene bank in pBR322. The original clone, containing a 6.5-kb insert, gave a functional product present in the periplasm of Escherichia coli. Analysis of polypeptides isolated after affinity chromatography on fibronectin-Sepharose followed by ion-exchange chromatography revealed two gene products, 87 and 165 kd in mol. wt. The amino acid compositions of these two polypeptides and a native FNBP from S. aureus strain Newman were very similar. Antibodies raised against the native FNBP from strain Newman precipitated the 125I-labelled 165-kd polypeptide, and unlabeled 165- and 87-kd polypeptides as well as native FNBP inhibited the immunoprecipitation reactions. The region of the fnbp-gene encoding the fibronectin-binding activity has been identified and subcloned in an expression vector based on the staphylococcal protein A gene. The resulting product in E. coli is an extracellular fusion protein consisting of two IgG-binding domains of protein A followed by a fibronectin-binding region. The fusion protein binds to fibronectin and completely inhibits the binding of fibronectin to intact cells of S. aureus.     

Messenger RNAs of a strongly-expressed late gene of cowpox virus contain 5''-terminal poly(A) sequences.

We have identified and characterized one of the most strongly-expressed genes of cowpox virus (CPV). This is the gene encoding the major protein component of the A-type inclusion bodies produced by this virus. This gene (designated the 160K gene) is transcribed late during the infection. Analyses of its mRNAs showed that these late RNAs, unlike all other characterized late mRNAs of poxviruses, are uniform in length. However, the most remarkable feature of the mRNAs of the 160K gene is the structure of their 5'-termini. Most of these mRNAs have 5'-terminal poly(A) sequences containing 5-21 residues. Furthermore, these 5'-terminal poly(A) sequences are not complementary to the corresponding region of the template strand of the viral DNA. Instead, the nucleotide sequences of the mRNA and the viral DNA diverge at the site of the three As in the sequence 5'-TAAATG-3' containing the gene's initiation codon. Consequently, the poly(A) provides the leader sequences of these mRNAs. These unusual 5'-terminal structures suggest that the late mRNAs of pox-virus genes are generated by a novel process.     

Structural and functional relationships between prokaryotic and eukaryotic DNA polymerases.

The Bacillus subtilis phage luminal diameter 29 DNA polymerase, involved in protein-primed viral DNA replication, was inhibited by phosphonoacetic acid (PAA), a known inhibitor of alpha-like DNA polymerases, by decreasing the rate of elongation. Three highly conserved regions of amino acid homology, found in several viral alpha-like DNA polymerases and in the luminal diameter 29 DNA polymerase, one of them proposed to be the PAA binding site, were also found in the T4 DNA polymerase. This prokaryotic enzyme was highly sensitive to the drugs aphidicolin and the nucleotide analogues butylanilino dATP (BuAdATP) and butylphenyl dGTP (BuPdGTP), known to be specific inhibitors of eukaryotic alpha-like DNA polymerases. Two potential DNA polymerases from the linear plasmid pGKL1 from yeast and the S1 mitochondrial DNA from maize have been identified, based on the fact that they contain the three conserved regions of amino acid homology. Comparison of DNA polymerases from prokaryotic and eukaryotic origin showed extensive amino acid homology in addition to highly conserved domains. These findings reflect evolutionary relationships between hypothetically unrelated DNA polymerases.