Efficient expression of small RNA polymerase III genes from a novel simian virus 40 vector and their effect on viral gene expression

Amber suppression in mammalian cells dependent upon expression of an Escherichia coli aminoacyl-tRNA synthetase gene

As an approach to inducible suppression of nonsense mutations in mammalian and in higher eukaryotic cells, we have analyzed the expression of an Escherichia coli glutamine-inserting amber suppressor tRNA gene in COS-1 and CV-1 monkey kidney cells. The tRNA gene used has the suppressor tRNA coding sequence flanked by sequences derived from a human initiator methionine tRNA gene and has two changes in the coding sequence. This tRNA gene is transcribed, and the transcript is processed to yield the mature tRNA in COS-1 and CV-1 cells. We show that the tRNA is not aminoacylated in COS-1 cells by any of the endogenous aminoacyl-tRNA synthetases and is therefore not functional as a suppressor. Concomitant expression of the E. coli glutaminyl-tRNA synthetase gene results in aminoacylation of the suppressor tRNA and its functioning as a suppressor. These results open up the possibility of attempts at regulated suppression of nonsense codons in mammalian cells by regulating expression of the E. coli glutaminyl-tRNA synthetase gene in an inducible, cell-type specific, or developmentally regulated manner.

Mechanism of interferon action: autoregulation of RNA-dependent P1/eIF-2 alpha protein kinase (PKR) expression in transfected mammalian cells

The expression of a molecular cDNA clone (P1 KIN) of the human RNA-dependent P1/eIF-2 alpha protein kinase (PKR) was examined in transfected monkey cells and in cell-free protein-synthesizing systems. Expression of the wild-type (wt) P1 KIN cDNA, which encodes an active protein kinase, was compared with that of the phosphotransfer catalytic domain II Lys-296-->Arg (K296R) mutant cDNA, which does not encode an active kinase. wt and K296R mutant P1 mRNAs prepared by transcription in vitro with T7 RNA polymerase programmed the cell-free synthesis of P1 ribosome-associated protein with comparable efficiency in the rabbit reticulocyte system. The K296R mutant P1 protein was also efficiently synthesized in vivo in transfected COS monkey cells. However, synthesis of the wt P1 protein was reduced about 30-fold in transfected COS cells as compared with the K296R mutant P1 protein. Cotransfection of wt P1 KIN cDNA with either K296R mutant P1 KIN cDNA or reovirus S4 cDNA greatly reduced the synthesis of K296R mutant P1 protein and reovirus sigma 3 protein, respectively. Although the wt and K296R mutant P1 KIN plasmid expression vectors replicated with comparable efficiencies in COS cells, the steady-state amount of P1 mRNA was about 3-fold less in COS cells transfected with the wt as compared with the K296R mutant P1 KIN cDNA. These results suggest that RNA-dependent P1 protein kinase expression is autoregulated in vivo in transfected mammalian cells primarily at the level of translation by a mechanism that is likely dependent upon catalytically active P1 kinase.

Analysis of alternatively spliced human immunodeficiency virus type-1 mRNA species, one of which encodes a novel tat-env fusion protein

A polymerase chain reaction-based analysis was used to define the structures of the mRNAs that encode human immunodeficiency virus type-1 (HIV-1) regulatory and structural proteins in infected H9 cells. Twenty alternatively spliced mRNAs encoding the vif, vpr, env, nef, tat, and rev proteins were characterized. An evaluation of the coding potentials of these transcripts recognized both leaky scanning and reinitiation at downstream initiation codons as mechanisms that may operate during translation of many of the polycistronic messages. Two new splice acceptor sites, one at nt 6018 defining a new mRNA coding for the env and vpu proteins and another at nt 8671 defining a novel tat-env fusion transcript, were characterized. The latter transcript expressed a novel protein p17tev that was immunoprecipitated by both polyclonal tat antibodies and monoclonals directed towards the C-terminal region of gp41. The p17tev protein was able to transactivate transcription from the HIV-1 LTR in transient transfection assays. The use of multiple alternative splice donor and acceptor sites and the generation of novel proteins may confer evolutionary advantages on the viral mutants encoding them and influence the course of clinical disease.