Procedures for enhancing the utility of the metallothionein promoter for the regulated expression of downstream open reading frames

Inducible expression bovine papillomavirus shuttle vectors containing ferritin translational regulatory elements

The combination of transcriptional and translational control elements in an inducible expression vector suitable for use in stably transformed cell lines was explored. To this end, ferritin translational control elements have been inserted downstream from a mouse metallothionein (mMT-I) transcriptional promoter (PmMT-I), and upstream from various reporter protein-encoding open reading frames (ORFs), all carried on a bovine papillomavirus shuttle vector. Protocols which stimulate transcription (with zinc) and translation (with iron) were developed to optimize the induction of reporter protein synthesis. It was found that insertion of an iron regulatory element between the PmMT-I and a reporter ORF bestowed a sixfold inducibility of reporter protein synthesis with iron and a 90-fold inducibility with iron plus zinc in a classical superinduction protocol. Surprisingly, inclusion of other rabbit ferritin light chain sequences (rFL), including the ORF, enhanced reporter inducibilities to over 15- and 500-fold, respectively. These additional rFL sequences not only increased inducibility but also (i) increased the half-life of the mRNA and (ii) strongly inhibited translation of an ORF located downstream from the 5' proximal ORF. The maximum levels of reporter proteins attained in transformed cells after prolonged induction represented from 1% to 7% of total cellular protein. These inducible expression vectors should prove useful for the production and study of cytotoxic proteins.

Characterization of a stable eukaryotic cell line expressing the Rous sarcoma virus integrase

The Rous sarcoma virus integration protein (IN) is required for efficient integration of viral DNA into the host genome. IN was expressed in mouse C127 cells using a bovine papillomavirus vector. This system utilizes the mouse metallothionein promoter and the SV40 late polyadenylation signal for efficient expression of IN. A stable cell line derived from a single hygromycin-resistant colony was characterized. The expression of IN increased significantly upon Zn2+ induction of the metallothionein promoter, but did not respond to "superinduction" protocols. Full-length nonphosphorylated IN was the major product of expression. A minor product resulting from initiation of translation at an internal Met codon was also produced. The expressed IN did not exhibit the polypeptide heterogeneity at its COOH-terminus nor phosphorylation as is seen when IN is immunoprecipitated from virions. Using subcellular fractionation and indirect immunofluorescence, IN was primarily localized to nuclei and in some cells appeared to concentrate at discrete loci within the nuclei.

Enhanced degradation of the ferritin repressor protein during induction of ferritin messenger RNA translation

Induction of ferritin synthesis in cultured cells by heme or iron is accompanied by degradation of the ferritin repressor protein (FRP). Intermediates in the degradative pathway apparently include FRP covalently linked in larger aggregates. The effect of iron on FRP degradation is enhanced by porphyrin precursors but is decreased by inhibitors of porphyrin synthesis, which implies that heme is an active agent. These results suggest that translational induction in this system may be caused by enhanced repressor degradation. While unique among translational regulatory systems, this process is common to a variety of other biosynthetic control mechanisms.

Specificity of the induction of ferritin synthesis by hemin

We have previously reported that hemin derepresses ferritin mRNA translation in vitro. As noted earlier, pre-incubation of a 90 kDa ferritin repressor protein (FRP) with hemin prevented subsequent repression of ferritin synthesis in a wheat germ extract. The significance of this observation has been investigated further. Evidence is presented here that this inactivation of FRP is temperature dependent. Neither FeCl3, Fe3+ chelated with EDTA, nor protoporphyrin IX caused significant inactivation of FRP under comparable conditions, whereas Zn2(+)-protoporphyrin IX produced an intermediate degree of inhibition. The presence of a glutathione redox buffer (GSB), which was previously shown to minimize non-specific side-effects of hemin, was not necessary for the derepression reaction. Inclusion of mannitol, a free radical scavenger, did not alter the inactivation caused by hemin. Calculation of the expected ratio of hemin monomers to dimers suggests that the active species is the monomer.

Inducible expression of encephalomyocarditis virus 3C protease activity in stably transformed mouse cell lines

An inducible expression vector system has been developed to facilitate the study of the effects of individual virus gene products on cell function. The vector utilizes the mouse metallothionein promoter carried on the bovine papillomavirus genome. Conditions which optimize the induced expression of open reading frames inserted downstream from the mouse metallothionein promoter have recently been described. In this communication we describe the use of this system to clone and express the encephalomyocarditis virus 3C protease in cultured mouse cells. Stably transformed cell lines could be induced to produce levels of 3C protease activity comparable to those observed during normal virus infection. In spite of this, no effects on cellular protein synthesis rate or membrane permeability were observed. It was also discovered that 3C protease as well as 3C protease-containing polyproteins are turned over. This was true not only in the induced cell clones, but also during the normal course of encephalomyocarditis virus infection, as well as in translation systems in vitro. This phenomenon was highly specific for this family of polypeptides, perhaps explaining their apparent lack of cytotoxic effects.

Metallothionein-I promoter-directed expression of foreign proteins in a mouse pituitary corticotrope tumor cell line

Expressing foreign proteins in heterologous eukaryotic cells has been a powerful tool for analyzing protein structure and function. The inducible mouse metallothionein-I promoter has been particularly useful for expression studies. However, the levels of expression achieved with this promoter in heterologous eukaryotic expression systems have not equaled those observed in vivo for the metallothionein-I gene. We have constructed expression plasmids placing either the gene for chloramphenicol acetyltransferase (CAT) or the cDNA for human neuropeptide Y (NPY) under control of the mouse metallothionein-I promoter. These two expression vectors were used to transfect mouse anterior pituitary tumor cells, from which stable transformants were isolated. The resulting cell lines, Mt.NPY1a and Mt.CAT, were used to maximize functional product expression from the metallothionein-I promoter. In both cell lines, a 35-fold induction of mRNA accumulation, peptide synthesis, or CAT activity was observed.

Translational repression in eukaryotes: partial purification and characterization of a repressor of ferritin mRNA translation

Mouse and rabbit ferritin mRNAs translate very poorly in rabbit reticulocyte lysates relative to most other mRNAs. This translational deficiency is not seen in wheat germ lysates, suggesting the presence of an inhibitor in reticulocyte lysate that is specific for ferritin mRNA. A specific repressor of ferritin mRNA translation has been partially purified from rabbit reticulocytes by differential ultracentrifugation, ammonium sulfate fractionation, and chromatography on phosphocellulose, DEAE-cellulose, and Sephacryl S-300. The elution profile from the latter suggests an aggregate molecular mass of approximately 180 kDa for the repressor. The inhibitory activity of this repressor against native ferritin mRNA can be relieved by adding in vitro transcripts of ferritin light-chain RNAs that contain the first 92 nucleotides of the 5' untranslated region. No other sequences appear to be necessary for this effect.