Thomas L. Mason

Heme regulates transcription of the CYC1 gene of S. cerevisiae via an upstream activation site

We show that expression of the iso-1-cytochrome c gene of Saccharomyces cerevisiae, CYC1, is tightly regulated by levels of intracellular heme. Expression is reduced at least 200-fold in cells grown under conditions of heme deficiency. Studies on the regulation of a CYC1-lacZ fused gene and direct determination of mRNA levels indicate that this control is transcriptional. Furthermore, we show that the heme regulatory site in the CYC1 promoter region is an upstream activation site (UASc) centered about 275 bp upstream from the region of transcriptional initiation. The latter region is required for optimal transcription and contains three TATA box sequences and six prominent mRNA initiation sites that span 34 bp. Substitution of the UASc with the UAS of the yeast GAL10 gene results in activation of the normal set of CYC1 transcripts. In this case, however, transcription is independent of regulation by heme, suggesting that in the wildtype, heme controls initiation per se and not translation or mRNA stability.

Japanese encephalitis virus-vaccinia recombinants produce particulate forms of the structural membrane proteins and induce high levels of protection against lethal JEV infection

Four recombinant vaccinia viruses were engineered for expression of different portions of the Japanese encephalitis virus (JEV) open reading frame. All four recombinant vaccinias contained the NS1 and NS2A genes, and each of these viruses specified the synthesis, glycosylation, and secretion of the nonstructural glycoprotein (NS1). All four recombinants also contained the E gene, and each virus correctly directed the synthesis and glycosylation of the envelope glycoprotein (E). Interestingly, two of these viruses (vP555 and vP650), which expressed the prM gene in addition to E and NS1, produced an extracellular hemagglutinin containing M and E that migrated in sucrose gradients similarly to the slowly-sedimenting hemagglutinin found in the culture fluid of JEV-infected cells. Immunization of 3-week-old mice with the recombinant viruses vP555 and vP658 resulted in immune responses to NS1, whereas only the virus that directed the synthesis of extracellular forms of E (vP555) induced an immune response to E. Both viruses provided protection against lethal challenge with JEV. Animals given two inoculations with vP555 were fully protected from greater than 10,000 LD50 of JEV. This high level of protection was correlated with the production of high titers of neutralizing and hemagglutination-inhibiting antibodies.

Smectic ordering in solutions and films of a rod-like polymer owing to monodispersity of chain length.

Solutions and melts of stiff (‘rod-like’) macromolecules often exhibit nematic liquid crystalline phases characterized by orientational, but not positional, molecular order,. Smectic phases, in which macromolecular rods are organized into layers roughly perpendicular to the direction of molecular orientation, are rare, owing at least in part to the polydisperse nature (distribution of chain lengths) of polymers prepared by conventional polymerization processes. Bacterial methods for polypeptide synthesis, in which artificial genes encoding the polymer are expressed in bacterial vectors, offer the opportunity to make macromolecules with very well defined chain lengths. Here we show that a monodisperse derivative of poly(γ-benzyl α,L-glutamate) prepared in this way shows smectic ordering in solution and in films. This result suggests that methods for preparing monodisperse polymers might provide access to new smectic phases with layer spacings that are susceptible to precise control on the scale of tens of nanometres.

Accumulation of mitochondrially synthesized Saccharomyces cerevisiae Cox2p and Cox3p depends on targeting information in untranslated portions of their mRNAs

The essential products of the yeast mitochondrial translation system are seven hydrophobic membrane proteins and Var1p, a hydrophilic protein in the small ribosomal subunit. Translation of the membrane proteins depends on nuclearly encoded, mRNA‐specific translational activators that recognize the 5′‐untranslated leaders of their target mRNAs. These translational activators are themselves membrane associated and could therefore tether translation to the inner membrane. In this study, we tested whether chimeric mRNAs with the untranslated sequences normally present on the mRNA encoding soluble Var1p, can direct functional expression of coding sequences specifying the integral membrane proteins Cox2p and Cox3p. DNA sequences specifying these chimeric mRNAs were inserted into mtDNA at the VAR1 locus and expressed in strains containing a nuclearly localized plasmid that supplies a functional form of Var1p, imported from the cytoplasm. Although cells expressing these chimeric mRNAs actively synthesized both membrane proteins, they were severely deficient in cytochrome c oxidase activity and in the accumulation of Cox2p and Cox3p, respectively. These data strongly support the physiological importance of interactions between membrane‐bound mRNA‐specific translational activators and the native 5′‐untranslated leaders of the COX2 and COX3 mRNAs for localizing productive synthesis of Cox2p and Cox3p to the inner membrane.

The antigenic structure of dengue type 1 virus envelope and NS1 proteins expressed in Escherichia coli

The antigenic structures of the envelope protein, E, and the non-structural protein, NS1, of dengue type 1 virus (DEN1) have been studied in the form of recombinant fusion proteins expressed in Escherichia coli. Deletion analysis was used to identify two distinct antigenic domains in E that reacted with subsets of antiviral monoclonal antibodies (MAbs). Domain I of E extends from amino acid residues (aa) 76 to 93 of E; domain II extends from aa 293 to 402 and contains an essential disulphide bridge. MAbs also reacted with several determinants clustered near the N terminus of the NS1 protein (aa 57 to 126). Recombinant fusion proteins containing E. coli trpE sequences and most of the sequences for either E or NS1 were immunogenic in mice. The antibodies elicited by the E fusion protein reacted with a portion of the protein containing domain II, whereas antibodies elicited by the NS1 fusion protein did not react with the antigenic determinants defined by our MAbs.

Cloning and characterization of the yeast chaperonin HSP60 gene.

The heat-shock protein, HSP60, is abundant in prokaryotes and eukaryotes and is required in the assembly of specific proteins. We have cloned the Saccharomyces cerevisiae HSP60 gene from a lambda gt11 genomic library using monoclonal antibodies, have obtained its sequence, determined its transcription start point, and shown that it exists as a single copy. The predicted HSP60 contains a mitochondrial target sequence and exhibits striking amino acid sequence similarity to its counterparts in bacteria, plants, and humans. These data indicate a high level of evolutionary conservation and are consistent with the suggestion of evolutionarily conserved function [Hemmingsen et al., Nature 333 (1988), 330-334].

Evidence that Synthesis of the Saccharomyces cerevisiae Mitochondrially Encoded Ribosomal Protein Var1p May Be Membrane Localized

ABSTRACT The 5′-untranslated leaders of mitochondrial mRNAs appear to localize translation within the organelle. VAR1 is the only yeast mitochondrial gene encoding a major soluble protein. A chimeric mRNA bearing the VAR1 untranslated regions and the coding sequence for pre-Cox2p appears to be translated at the inner membrane surface. We propose that translation of the ribosomal protein Var1p is also likely to occur in close proximity to the inner membrane.

A single nucleotide substitution at the rib2 locus of the yeast mitochondrial gene for 21S rRNA confers resistance to erythromycin and cold-sensitive ribosome assembly

SummaryWe have studied a mutation (cs23) in the mitochondrial gene for 21SrRNA that affects the peptidyl transferase center of the ribosome and conditionally blocks the assembly of the 54S ribosomal subunit. Strains carrying this mutation are resistant to erythromycin and cold-sensitive for growth on nonfermentable carbon sources (Singh et al. 1978) Mitochondria isolated from mutant cells grown on glucose at 20°C, the nonpermissive temperature, were depleted of the 54S subunit and instead contained a novel 45S ribosomal particle. After mutant cells were shifted from 20°C to 32°C, 54S subunits were assembled, apparently from the 45S particles and pre-existing ribosomal proteins. DNA sequencing revealed that the mutant phenotype is a consequence of a C to A transversion at position 3993 of the 21SrRNA gene. Previously, C to U and C to G mutations have been identified at the same position in the 21S rRNA sequence. This position corresponds to C-2611 in the E. coli 23S RNA, a nucleotide that appears to be conserved in the large rRNA of all erythromycin-sensitive ribosomes.

Biosynthetic Incorporation and Chemical Modification of Alkene Functionality in Genetically Engineered Polymers

Repetitive polypeptides of sequence [(AlaGly)_3ProGluGly]_(16) 3a, have been prepared in Escherichia coli as overexpressed recombinant proteins. Replacement of more than 90% of the naturally occurring proline (Pro) residues with 3,4-dehydroproline (Dhp) in sequence 3a was achieved by in vivo expression of the target protein in medium containing Dhp and lacking Pro. The resulting material (3b) was treated with H_2O_2 or Br_2 to yield polymers containing 3,4-dihydroxyproline (Dhy, 3c) and 3,4-dibromoproline (Dbr, 3d), respectively, in place of the Dhp residue. These results represent the first demonstration of the incorporation and modification of alkene functionality in recombinant proteins.

Protein engineering for materials applications

The advent of recombinant DNA technology has provided new approaches to the synthesis of proteins with periodic structures. Reports are now emerging that describe the implementation of this technology for protein engineering of natural periodic proteins and for the de novo design of protein-like polymeric materials.