A functional hybrid ribosome binding site in tryptophan operon messenger RNA of Escherichia coli

Functional expression of plant acetolactate synthase genes in Escherichia coli

Acetolactate synthase (ALS; EC 4.1.3.18) is the first common enzyme in the biosynthetic pathways leading to leucine, isoleucine, and valine. It is the target enzyme for three classes of structurally unrelated herbicides, the sulfonylureas, the imidazolinones, and the triazolopyrimidines. A cloned ALS gene from the small cruciferous plant Arabidopsis thaliana has been fused to bacterial transcription/translation signals and the resulting plasmid has been used to transform Escherichia coli. The cloned plant gene, which includes sequences encoding the chloroplast transit peptide, is functionally expressed in the bacteria. It is able to complement genetically a strain of E. coli that lacks endogenous ALS activity. An ALS gene cloned from a line of Arabidopsis previously shown to be resistant to sulfonylurea herbicides has been similarly expressed in E. coli. The herbicide-resistance phenotype is expressed in the bacteria, as assayed by both enzyme activity and the ability to grow in the presence of herbicides. This system has been useful for purifying substantial amounts of the plant enzyme, for studying the sequence parameters involved in subcellular protein localization, and for characterizing the interactions that occur between ALS and its various inhibitors.

Expression and site-specific mutagenesis of the poliovirus 3C protease in Escherichia coli

We have engineered a segment of the poliovirus genome (nucleotides 5438-6061) that encodes the 183 amino acid residues of the 3C region and 25 residues of the 3D region of the viral polyprotein into an Escherichia coli expression vector. The 3C region is a virus-specific protease, which, when expressed in E. coli, is shown to be active and autocatalytic. In our system, three poliovirus-specific proteins are produced: a precursor polyprotein (3C-3D), an internal initiation product, and the mature protease (3C). Mutants in the 3C region have been constructed by oligonucleotide-directed mutagenesis and their effect on the proteolytic activity has been assayed by the in vivo production of the mature protease. The mutation of highly conserved residues (cysteine-47 or histidine-161) produced an inactive enzyme, while the mutation of a nonconserved residue (cysteine-153) had a negligible effect on the proteolytic activity.

The effects of hybrid ribosome-binding-site variants on the expression of human interferon-beta in Escherichia coli

Human beta-interferon (IFN-beta) has been expressed in Escherichia coli by using vectors that join the trp promoter and a hybrid ribosome-binding site (HRBS) to the mature IFN-beta coding sequence. Introduction of an NcoI site at the ATG initiation codon of IFN-beta by site-directed mutagenesis has facilitated the construction of a series of portable HRBSs, by utilizing oligodeoxynucleotide adapters. The spacing between the Shine-Dalgarno (S-D) sequence and the initiator ATG ranged from 7-13 bp. One spacer of 9 bp length was varied at a single position. IFN-beta expression by these HRBS variants, as analyzed by antiviral activity and SDS-PAGE, shows both nucleotide sequence and spacer length effects. In vitro transcription-translation experiments indicate that some single base changes in the HRBS significantly decrease the rate of translation of the IFN-beta mRNA.

Studies on the interaction of Trp holorepressor with several operators. Evidence that the target need not be palindromic

The interaction of Trp repressor protein with partial trp operators was studied in vitro and in vivo. At high ratios of protein to DNA, Trp holorepressor formed stable complexes with DNA molecules containing half operators. When plasmids conferring the capacity to hyperproduce Trp repressor were present in trpOc strains of Escherichia coli, repression of downstream tryptophan synthase occurred. Palindromicity of the trp operator may facilitate stable interaction with Trp repressor, but this attribute need not be regarded as a critically essential structural feature. Sufficient information for the recognition by Trp repressor protein of an appropriate target resides within a DNA sequence of approximately ten base-pairs.

The construction of a synthetic Escherichia coli trp promoter and its use in the expression of a synthetic interferon gene

An 82 base pair DNA fragment has been synthesised which contains the E. coli trp promoter and operator sequences and also encodes the first Shine Dalgarno sequence of the trp operon. This DNA fragment is flanked by EcoRI and ClaI/TaqI cohesive ends and is thus easy to clone, transfer between vector systems and couple to genes to drive their expression. It has been cloned into plasmid pAT153, producing a convenient trp promoter vector. We have also joined the fragment to a synthetic IFN-alpha 1 gene, using synthetic oligonucleotides to generate a completely natural, highly efficient bacterial translation initiation signal on the promoter proximal side of the IFN gene. Plasmids carrying this construction enable E. coli cells to express IFN-alpha 1 almost constitutively and with significantly higher efficiency than from a lacUV5 promoter based system.

The complete nucleotide sequence of the tryptophan operon of Escherichia coli

The tryptophan (trp) operon of Escherichia coli has become the basic reference structure for studies on tryptophan metabolism. Within the past five years the application of recombinant DNA and sequencing methodologies has permitted the characterization of the structural and functional elements in this gene cluster at the molecular level. In this summary report we present the complete nucleotide sequence for the five structural genes of the trp operon of E. coli together with the internal and flanking regions of regulatory information.

Synthetic sites for transcription termination and a functional comparison with tryptophan operon termination sites in vitro

Termination of transcription by Escherichia coli RNA polymerase in vitro appears to depend primarily on two structural features of the termination site--a G+C-rich region of dyad symmetry and a series of terminal uridine residues in the transcript. To determine whether these two features are sufficient to specify rho-independent termination in vitro, we have introduced new sequences within a tryptophan (trp) operon structural gene to create two sites with these characteristics. Transcription with wild-type RNA polymerase in vitro demonstrates that discrete termination occurs at one of these new sites, although at a low level. Use of the mutant RNA polymerase rpo203, which is more sensitive to certain weak terminators than is the wild-type enzyme, increases termination at both sites. We have compared the activity of our synthetic terminators with those of several termination sites in the E. coli trp operon. Under normal conditions of transcription in vitro, termination becomes more efficient with an increase in the length of the stem in the RNA hairpin or an increase in the number of consecutive uridine residues. Transcription with the rpo203 polymerase and with ribonucleotide analogs gives changes consistent with these general trends. These results support a model for termination involving separate but essential roles for the RNA hairpin and the stretch of uridines in the transcript.

Tandem termination sites in the tryptophan operon of Escherichia coli

In vivo, transcription of tryptophan (trp) operon mRNA appears to terminate at a site (trp t) 36 nucleotides after the last structural gene, and efficient function at this site requires the protein factor rho. However, distal nucleotide sequences also seem to play a role in modulating termination at trp t. We report here our in vitro studies of DNA fragments carrying portions of the trp termination region. Transcription of these DNA fragments in a purified system demonstrates that RNA polymerase actually recognizes two different termination sites. Termination at the previously characterized site, trp t, is only 25% efficient, and it is unaffected by the presence of rho factor in vitro. However, addition of rho to the transcription reaction mixture reveals that termination also occurs within a region that we have designated trp t', located about 250 bases past trp t. These two sites behave independently in vitro, whether in the tandem configuration or cloned separately, and their structural features and functional characteristics are quite different. This contrasts with the observation that termination of transcription at the end of the trp operon in vivo appears to require a rho-mediated interaction between trp t and trp t'. The possible involvement of other factors and the significance of multiple termination sites is discussed.