Role of the avian retrovirus mRNA leader in expression: evidence for novel translational control

Cross- and Co-Packaging of Retroviral RNAs and Their Consequences

Retroviruses belong to the family Retroviridae and are ribonucleoprotein (RNP) particles that contain a dimeric RNA genome. Retroviral particle assembly is a complex process, and how the virus is able to recognize and specifically capture the genomic RNA (gRNA) among millions of other cellular and spliced retroviral RNAs has been the subject of extensive investigation over the last two decades. The specificity towards RNA packaging requires higher order interactions of the retroviral gRNA with the structural Gag proteins. Moreover, several retroviruses have been shown to have the ability to cross-/co-package gRNA from other retroviruses, despite little sequence homology. This review will compare the determinants of gRNA encapsidation among different retroviruses, followed by an examination of our current understanding of the interaction between diverse viral genomes and heterologous proteins, leading to their cross-/co-packaging. Retroviruses are well-known serious animal and human pathogens, and such a cross-/co-packaging phenomenon could result in the generation of novel viral variants with unknown pathogenic potential. At the same time, however, an enhanced understanding of the molecular mechanisms involved in these specific interactions makes retroviruses an attractive target for anti-viral drugs, vaccines, and vectors for human gene therapy.

The critical role of proximal gag sequences in feline immunodeficiency virus genome encapsidation

Retroviral RNA encapsidation is mediated by specific interactions between viral Gag proteins and cis-acting packaging sequences in genomic RNA. Feline immunodeficiency virus (FIV) RNA encapsidation determinants have been shown to be discrete and noncontinuous, comprising one region at the 5' end of the genomic mRNA (R-U5) and another region that mapped within the proximal 311 nt of gag. To aid comparative understanding of lentiviral encapsidation and refinement of FIV vector systems, we used RNase protection assays (RPAs) of cellular and virion RNAs to investigate in detail the gag element. mRNAs of subgenomic vectors as well as of full-length molecular clones were optimally packaged into viral particles and resulted in high-titer FIV vectors when they contained only the proximal 230 nucleotides (nt) of gag. Further 3' truncations of gag sequences progressively diminished encapsidation and transduction. Deletion of the initial ninety 5' nt of the gag gene abolished mRNA packaging, demonstrating that this segment is indispensable for encapsidation. Focusing further on this proximal sequence, we found that a deletion of only 13 nt at the 5' end of gag impaired encapsidation of subgenomic vector and proviral RNAs.

The role of the 5' nontranslated regions of the fusion protein mRNAs of canine distemper virus and rinderpest virus

The mRNAs which code for the fusion proteins of the morbilliviruses (measles virus, canine distemper virus, and rinderpest virus) have unusually long 5' untranslated regions (UTRs) which are GC-rich and are capable of folding into extensive secondary structures. In measles virus the first AUG codons in the fusion (F) protein mRNA are in close proximity at nucleotide positions 574 and 583 and protein translation is initiated at the second position. In the canine distemper virus (CDV) and rinderpest virus (RPV) F gene transcripts the analogous initiation codons are preceded by several other AUG codons many nucleotides upstream either in the same reading frame or at the beginning of other short open reading frames. We have studied the effect of deleting these upstream regions on the production of the fusion proteins of both CDV and RPV from cDNA constructs. Within the cells the presence of these regions enhances the production of the F protein while, in contrast, the production of the authentic F protein from in vitro translations using RNA transcripts is inhibited by these sequences.

Generation of a helper cell line for packaging avian leukosis virus-based vectors

We constructed an avian leukosis virus-based packaging cell line, pHF-g, containing Rous-associated virus DNA with several alterations to abolish RNA packaging. One of them is a 52-base-pair deletion encompassing the putative encapsidation signal in the leader region. The 3' long terminal repeat was also removed and replaced by the polyadenylation sequence from the herpes simplex virus thymidine kinase gene. When pHF-g cells were transfected by an avian leukosis virus-based vector, they produced replication-defective virus at high titer but they did not release any replication-competent particles. Proviral DNA was shown to be correctly integrated as well as correctly expressed. Viral RNAs were shown to be correctly translated into gag-related polypeptides.

HIV-1 expression is posttranscriptionally repressed in Drosophila cells

The long terminal repeats (LTRs) of the human immunodeficiency virus (HIV) the Rous sarcoma virus (RSV) and the copia Drosophila retrotransposon were compared in their capacity to direct expression of the bacterial cat (chloramphenicol acetyltransferase) gene in human, murine, and Drosophila cell lines. The results indicate that HIV and RSV LTR expression is post transcriptionally repressed in the Drosophila cells while copia LTR expression is post-transcriptionally repressed in the human and murine cells.

Polycistronic animal virus mRNAs

This chapter discusses some observations concerning the natural occurrence and structural organization of polycistronic animal virus mRNAs, and the mechanisms by which they may be translated to yield two or more unique polypeptide products. In most polycistronic viral mRNAs, initiation of translation of both the 5’-proximal, upstream cistron and the internal, downstream cistron(s) likewise occurs at an AUG codon. Animal viruses encoding polycistronic mRNAs in which translation-initiation occurs alternatively at one or more AUG initiation sites, include members of several virus families that utilize a variety of different replication strategies as parts of their life cycles. They include: 1. viruses with DNA genomes and viruses with RNA genomes; 2. viruses with circular genomes and viruses with linear genomes; 3. viruses whose genomes are constituted by a single piece of nucleic acid, as well as viruses with segmented genomes; and 4. viruses that utilize the cell nucleus as the site for mRNA biogenesis, as well as viruses whose mRNA is synthesized in the cytoplasm. Furthermore, many different biochemical mechanisms may exist in animal cells to permit the expression of functionally polycistronic viral mRNAs.

Cauliflower mosaic virus 35 S RNA leader region inhibits translation of downstream genes

The cauliflower mosaic virus (CaMV) 35 S RNA is a full-length transcript of the viral genome. It encodes the genes VII and I-V, arranged in tandem along the RNA, preceded by a long leader region (600 bases) containing many short open reading frames. We have examined the effects of the leader and the first gene (gene VII) on downstream gene I translation in vitro and in an in vivo transient expression system (carrot protoplasts). RNAs from constructs containing the intact leader, and from various deletion constructs, were translated in a rabbit reticulocyte system. Gene I was translated efficiently only when the long leader region and the upstream gene VII were deleted. Translational fusions of gene VII or I to the firefly luciferase reporter gene were also constructed, and a similar series of leader sequence deletion mutants were examined in vivo and in vitro. The 600-base leader region was found to repress translation of gene VII 8- to 30-fold as compared to the truncated gene lacking the leader region. Gene I expression as compared to that of gene VII was reduced an additional 7- to 20-fold by the presence of the upstream leader region including gene VII. This represented an overall reduction in gene I expression of greater than 100-fold as compared to expression in the absence of any leader sequence. The reduced translation of gene I in the context of the 35 S RNA leader region was not due to the action of the gene VII protein product but may result from efficient blocking of scanning 40 S ribosomes by translation of upstream open reading frames.

Expression of a cloned human interleukin-2 cDNA is enhanced by the substitution of a heterologous mRNA leader region

A cloned interleukin-2 (IL-2) cDNA was poorly expressed in transfected eukaryotic cells. This low expression was only partly relieved by removal of the homopolymer tail present in the 5' leader region of the encoded IL-2 mRNA. However, replacement of the natural IL-2 mRNA 5' noncoding region with a leader element derived from the efficiently translated rat preproinsulin II mRNA resulted in an mRNA molecule that was utilized effectively by the transfected cell. The enhanced expression of this chimeric IL-2 mRNA did not appear to be due to changes in the sequence near the translation initiation codon. These results suggest that the leader elements of efficiently translated mRNAs may be able to confer a higher translational efficiency on heterologous protein coding regions when present in cis.

Expression of the human cardiac actin gene in differentiating rat skeletal myoblasts

The human cardiac-actin (CH-actin) gene was transfected into rat L6 skeletal myoblasts and stable transformants were isolated. The level of the CH-actin transcript varied between clones but changed little during the differentiation of myoblasts into multinucleate myotubes. Chimeric genes were constructed in which the CH-actin promoter, first non-coding exon (44 bp), and first intron (about 700 bp) were linked to the Herpes simplex virus thymidine kinase (tk) coding region. Clones of L6 cells transformed with these chimeric genes contained variable levels of actin-tk mRNA which changed little during differentiation. Thus, the activity of the CH-actin promoter appeared not to be up-regulated upon differentiation of myoblasts into myotubes. In clones of cells expressing the actin-tk mRNA, the TK protein was not detected in myoblasts but appeared in differentiating multinucleate myotubes. We interpret these results as suggesting developmentally regulated translation of the actin-tk mRNA. Since the first 44 nucleotides of the actin-tk mRNA were derived from the 5'-untranslated region of the CH-actin mRNA. These experiments suggest that translation of the actin-tk mRNA may be controlled by this region.

Secondary structure and expression in vivo and in vitro of messenger RNAs into which upstream AUG codons have been inserted

We wanted to discover whether the conformation of the mRNA leader sequence is involved in translational fidelity. For this purpose we constructed several mutants of Semliki Forest virus 26S mRNA and inserted AUG codons into the leader sequence. We then analyzed the results of in vitro and in vivo translation of these mRNAs, probed enzymatically the secondary structure and performed minimal energy folding of the transcripts. Our results indicate that the position of a hairpin in the leader sequence determines at which AUG codon downstream from that hairpin translation is initiated.

Three types of muscle-specific gene expression in fusion-blocked rat skeletal muscle cells: translational control in EGTA-treated cells

When rat skeletal muscle cells were treated with EGTA, an inhibitor of cell fusion, a battery of muscle-specific mRNAs was synthesized but not translated despite the synthesis of many other proteins. Most of the muscle-specific mRNAs were associated with polysomes in fused myotubes, whereas they were found in postpolysomal fractions in EGTA-treated cells. Therefore, in addition to the well-documented transcriptional and posttranscriptional control of muscle-specific genes, translational control of this specific group of genes, presumably involving a Ca2+-dependent process, is also observed in these fusion-blocked cells. These findings and results obtained with other fusion inhibitors demonstrate that three types of muscle-specific gene expression take place in the fusion-blocked cells depending on the inhibitors used: one, neither muscle-specific mRNAs nor proteins are synthesized; two, the mRNAs are synthesized but not translated; and three, both the mRNAs and the proteins are synthesized.

Trans-activation of human immunodeficiency virus occurs via a bimodal mechanism

A novel, highly quantitative transient expression assay based on the human interleukin-2 (IL-2) gene was used to examine the trans-activation of the Human Immunodeficiency Virus (HIV/HTLV-III/LAV/ARV) long terminal repeat (LTR) in a range of eukaryotic cell lines. In the absence of the trans-activating viral gene product, tat-III, IL-2 transcripts specific for the HIV LTR were present in low abundance in transfected cells and showed a low translational efficiency, when compared with IL-2 mRNAs transcribed from other viral promoters. Coexpression of tat-III resulted in a marked increase in the steady state level of IL-2 mRNAs transcribed from the HIV LTR, and these mRNAs also demonstrated a specific enhancement of their translational efficiency. These results suggest a bimodal mechanism of action for tat-III in the trans-activation of HIV-specific gene expression.

Gene insertion into the chicken germ line by retroviruses

We injected chick syncytial strain of reticuloendotheliosis virus (CS-REV) and wild type and recombinant avian leukosis virus (ALV) near the blastoderm of unincubated fertilized embryos and CS-REV intra-abdominally at day of hatch, and we progeny tested the surviving ALV viremic males and REV viremic males and females for transmitted viral genetic material. A number of positive progeny were identified and their deoxyribonucleic acid (DNA) analyzed for restriction enzyme fragments that hybridized with viral genetic material. Most of the progeny had simple restriction enzyme patterns unlike the viremic parents or congenitally infected progeny. This is suggestive evidence that retroviral genetic information has been inserted into the germ line of chickens.

Regulation of protein synthesis in virus-infected animal cells

This chapter summarizes the structural features that govern the translation of viral mRNAs: where the synthesis of a protein starts and ends, how many proteins can be produced from one mRNA, and how efficiently. It focuses on the interplay between viral and cellular mRNAs and the translational machinery. That interplay, together with the intrinsic structure of viral mRNAs, determines the patterns of translation in infected cells. It also points out some possibilities for translational regulation that can only be glimpsed at present, but are likely to come into focus in the future. The mechanism of selecting the initiation site for protein synthesis appears to follow a single formula. The translational machinery displays a certain flexibility that is exploited more frequently by viral than by cellular mRNAs. Although some of the parameters that determine efficiency have been identified, how efficiently a given mRNA will be translated cannot be predicted by summing the known parameters.