Suppression of murine retrovirus polypeptide termination: effect of amber suppressor tRNA on the cell-free translation of Rauscher murine leukemia virus, Moloney murine leukemia virus, and Moloney murine sarcoma virus 124 RNA

Reverse Transcriptase of Moloney Murine Leukemia Virus Binds to Eukaryotic Release Factor 1 to Modulate Suppression of Translational Termination

The pol (for polymerase) gene of the murine leukemia viruses (MuLVs) is expressed in the form of a large Gag-Pol precursor protein by the suppression of translational termination, or enhanced readthrough, of a UAG stop codon at the end of gag. A search for cellular proteins that interact with the reverse transcriptase of Moloney MuLV resulted in the identification of eRF1, the eukaryotic translation release factor 1. The proteins bound strongly in vitro, and the overexpression of eRF1 resulted in the RT-dependent incorporation of the protein into assembling virion particles. The overexpression of RT in trans enhanced the translational readthrough of a reporter construct containing the Gag-Pol boundary region. Noninteracting mutants of RT failed to synthesize adequate levels of Gag-Pol and could not replicate. These results suggest that RT enhances suppression of termination and that the interaction of RT with eRF1 is required for an appropriate level of translational readthrough.

Mutational analysis of the gag-pol junction of Moloney murine leukemia virus: requirements for expression of the gag-pol fusion protein

The gag-pol polyprotein of the murine and feline leukemia viruses is expressed by translational readthrough of a UAG terminator codon at the 3' end of the gag gene. To explore the cis-acting sequence requirements for the readthrough event in vivo, we generated a library of mutants of the Moloney murine leukemia virus with point mutations near the terminator codon and tested the mutant viral DNAs for the ability to direct synthesis of the gag-pol fusion protein and formation of infectious virus. The analysis showed that sequences 3' to the terminator are necessary and sufficient for the process. The results do not support a role for one proposed stem-loop structure that includes the terminator but are consistent with the involvement of another stem-loop 3' to the terminator. One mutant, containing two compensatory changes in this stem structure, was temperature sensitive for replication and for formation of the gag-pol protein. The results suggest that RNA sequence and structure are critical determinants of translational readthrough in vivo.

Bipartite signal for read-through suppression in murine leukemia virus mRNA: an eight-nucleotide purine-rich sequence immediately downstream of the gag termination codon followed by an RNA pseudoknot

The pol gene of murine leukemia virus and other mammalian type C retroviruses is expressed by read-through suppression of an in-frame UAG codon which separates the gag and pol coding regions. In this study, we have analyzed the sequence requirements for read-through suppression by placing different portions of wild-type and mutant viral sequences from the gag-pol junction between reporter genes and testing transcripts of these constructs for suppression in reticulocyte lysates. We find that the read-through signal is contained within the first 57 nucleotides on the 3' side of the UAG codon. Our results indicate that the identities of six conserved bases in the eight-nucleotide, purine-rich sequence immediately downstream of the UAG codon are critical for suppression, as is the existence of a pseudoknot structure spanning the next 49 nucleotides. Thus, read-through suppression depends on a complex, bipartite signal in the mRNA.

Translational suppression in retroviral gene expression

This chapter summarizes the present state of knowledge concerning translational suppression in retroviruses. Other viruses, using similar mechanisms, are mentioned only briefly and tangentially. Retroviruses are a unique class of viruses that have been found in all classes of vertebrates but not in other organisms. Perhaps, their most distinctive properties are the flow of information from RNA to DNA early in the infectious process, and the subsequent integration of the viral DNA into the chromosomal DNA of the host cell. Retroviruses are the causative agents of acquired immunodeficiency syndrome (AIDS) and of a variety of neoplastic diseases in man and domestic animals. Elements with striking similarities to retroviruses, termed retrotransposons, occur in yeast and many other eukaryotes; elements sharing some characteristics with retroviruses have also recently been observed in prokaryotes. Because of the apparent relationship between retroviruses and retrotransposons, this chapter discusses of retrotransposons as well as retroviruses. Though all retroviruses utilize translational suppression in pol-protein synthesis, different groups of retroviruses use two completely distinct types of translational suppression. One of these is in-frame or readthrough suppression and the other is ribosomal frameshifting.

Evidence that a downstream pseudoknot is required for translational read-through of the Moloney murine leukemia virus gag stop codon

Approximately 5% of the ribosomes translating the gag gene of murine leukemia viruses read through the UAG terminator and translate the in-frame pol gene to produce the gag-pol fusion polyprotein, the sole source of the pol gene products. We show that a pseudoknot located eight nucleotides 3' of the UAG codon in the Moloney murine leukemia virus is required for read-through. This requirement is markedly different from that known to be involved in other cases of read-through but surprisingly similar to some stimulatory sequences known to promote ribosomal frameshifting.

cis Acting RNA sequences control the gag-pol translation readthrough in murine leukemia virus

The pol gene of the Moloney murine leukemia virus (M-MuLV) is expressed as a Gag-Pol fusion protein through an in-frame suppression of the UAG termination codon located between the two genes. The role of nucleotide context in suppression was investigated, in a rabbit reticulocyte lysate translation system, using site-directed mutagenesis. The results indicate that the translational readthrough is mediated by at least 50 bases long RNA sequence located 3' to the gag UAG termination codon. Within this sequence a short purine-rich sequence adjacent to the amber codon, highly conserved among different retroviruses, appears essential for M-MuLV suppression. Two alternative putative stem and loop like RNA structures can be drawn at the gag-pol junction, one abutting the gag UAG codon, and the second downstream to it. None of these structures appears to be important to the suppression process.

Identification of amino acids inserted during suppression of UAA and UGA termination codons at the gag-pol junction of Moloney murine leukemia virus

Expression of the murine leukemia virus pol gene occurs by translational readthrough of an in-frame UAG codon between the gag and pol coding regions. In a previous study, we mutated the UAG codon to UAA or UGA and demonstrated that both of these termination codons could be suppressed in reticulocyte lysates and in infected cells with the same efficiency as UAG. We now report the identity of the amino acids inserted in vitro in response to UAA and UGA in fusion products containing the gag-pol junction region. The results show that UAA, like UAG, directs the incorporation of glutamine, whereas UGA directs the incorporation of three amino acids, arginine, cysteine, and tryptophan. To our knowledge, this is the first report indicating misreading of UAA as glutamine and UGA as arginine and cysteine in higher eukaryotes. Interestingly, although our protein synthesis system presumably contains other known UAG and UGA suppressors, these tRNAs did not suppress the termination codons in our experiments. Thus, it seems possible that the sequence surrounding the gag-pol junction not only promotes suppression but also helps determine which tRNAs function in suppression.

Suppression of UAA and UGA termination codons in mutant murine leukemia viruses

Genomes of mammalian type C retroviruses contain a UAG termination codon between the gag and pol coding regions. The pol region is expressed in the form of a gag-pol fusion protein following readthrough suppression of the UAG codon. We have used oligonucleotide-directed mutagenesis to change the UAG in Moloney murine leukemia virus to UAA or UGA. These alternate termination codons were also suppressed, both in infected cells and in reticulocyte lysates. Thus, the signal or context inducing suppression of UAG in wild-type Moloney murine leukemia virus is also effective with UAA and UGA. Further, mammalian cells and cell extracts contain tRNAs capable of translating UAA and UGA as amino acids. To our knowledge, this is the first example of natural suppression of UAA in higher eucaryotes.

Translational readthrough of the murine leukemia virus gag gene amber codon does not require virus-induced alteration of tRNA

An in vitro system to assay translational readthrough of the UAG termination codon at the murine leukemia virus (MuLV) gag-pol junction was developed by using rabbit reticulocyte lysates programmed by SP6-generated Moloney MuLV gag-pol mRNA. Under conditions in which the suppressor activity of the lysate was dependent on addition of tRNA, it could be shown that readthrough synthesis was stimulated to approximately the same extent by equivalent amounts of tRNA from MuLV-infected and uninfected NIH 3T3 cells. Analysis of glutamine tRNA, which mediates suppression in vivo, showed that the level of glutamine acceptor activity and the chromatographic profile of glutamine isoacceptors were unchanged following virus infection. On the basis of these results, we conclude that the suppressor tRNA occurs normally within the tRNA population of uninfected cells and need not be induced in response to virus infection.

Stop making sense: or Regulation at the level of termination in eukaryotic protein synthesis

An increasing number of examples of translational regulation at the level of termination has been recently reported in eukaryotes. This paper reviews our present knowledge on this topic and proposes an understanding of these regulations by relating the study of viral gene expression to a comprehensive view of the mechanisms and components of the translational process.

Nucleotide sequence of a radiation leukemia virus genome

The complete nucleotide sequence of an infectious molecular clone of a radiation murine leukemia proviral DNA RadLV/VL3(T+L+) has been determined. The sequence of the RNA genome is 8318 nucleotides long and contains three large open reading frames encoding the gag, pol, and env gene products. With the exception of a xenotropiclike R peptide and the LTR which bears structural similarities to a xenotropic LTR, displaying typical enhancerlike sequences, the remaining sequences are strikingly similar to the endogenous, ecotropic Akv murine leukemia virus. Therefore, it could be postulated that the leukemogenic properties of RadLV/VL3(T+L+) were generated by a recombination event between a xenotropic virus and an Akv-like ecotropic virus.

Persistent expression of v-mos oncogene in transformed cells that revert to nonmalignancy after prolonged treatment with interferon

BALB/c embryonic fibroblasts productively transformed by Moloney sarcoma virus and cultivated for over 600 generations in the presence of mouse alpha/beta interferon reverted to an apparently normal phenotype and were unable to produce tumors in nude mice. Nevertheless, the presence of an integrated Moloney sarcoma virus genome in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA could be shown by focus formation upon transfection and by hybridization with a v-mos probe. After digestion with various restriction endonucleases, similar hybridization patterns of v-mos sequences were obtained with DNAs from both reverted and transformed cells. However, additional integration sites and at least twice as many copies of the oncogene were found in the nonmalignant Moloney sarcoma virus-transformed interferon-treated cell DNA. Polyadenylylated RNA extracted from reverted and control cells contained two mos-specific transcripts. Interestingly, the nonmalignant Moloney sarcoma virus-transformed interferon-treated cells produced helper virus, but no detectable mos-containing virions, suggesting that a posttranscriptional block in the v-mos gene expression had occurred in these cells. It should be stressed that, after up to 100 additional passages, cells cultured in the absence of interferon maintained their nontumorigenic character in spite of the persistent transcription of the mos oncogene.

Suboptimal growth with hyper-accurate ribosomes

Mutant bacteria with hyperaccurate ribosomes support their excessive accuracy of translation in vitro by dissipating 1.5 to 2.5 cognate ternary complexes per peptide bond formed. This is to be compared with a dissipation rate close to 1.1 for wild-type ribosomes. Here, we have tested the hypothesis that a corresponding loss of translational efficiency in vivo would lower the growth rate of the mutants. Such a growth inhibitory effect would explain why the lower accuracy of wild-type ribosomes is more fit. Our data show that as expected the mu of the hyperaccurate mutants is smaller than that of wild-type bacteria. In contrast, during glucose-limited growth in chemostats there is not the same simple correlation between growth yield and ribosomal efficiency for the hyperaccurate mutants.

The pathophysiology of murine retrovirus-induced leukemias

Murine leukemia viruses (MuLVs) are retroviruses which induce a broad spectrum of hematopoietic malignancies. In contrast to the acutely transforming retroviruses, MuLVs do not contain transduced cellular genes, or oncogenes. Nonetheless, MuLVs can cause leukemias quickly (4 to 6 weeks) and efficiently (up to 100% incidence) in susceptible strains of mice. The molecular basis of MuLV-induced leukemia is not clear. However, the contribution of individual viral genes to leukemogenesis can be assayed by creating novel viruses in vitro using recombinant DNA techniques. These genetically engineered viruses are tested in vivo for their ability to cause leukemia. Leukemogenic MuLVs possess genetic sequences which are not found in nonleukemogenic viruses. These sequences control the histologic type, incidence, and latency of disease induced by individual MuL Vs.

Nucleotide sequence of AKV murine leukemia virus

AKV is an endogenous, ecotropic murine leukemia virus that serves as one of the parents of the recombinant; oncogenic mink cell focus-forming viruses that arise in preleukemic AKR mice. I report the 8,374-nucleotide-long sequence of AKV, as determined from the infectious molecular clone AKR-623. The 5'-leader sequence of AKV extends to nucleotide 639, after which lies a long open reading frame encoding the gag and pol gene products. The reading frame is interrupted by a single amber codon separating the gag and pol genes. The pol gene overlaps the env gene within the 3' region of the AKV genome. The nucleotide sequence of the 5' region of AKV reveals the following features. (i) The 5'-leader sequence lacks any AUG codon to initiate translation of gPr80gag, suggesting that gPr80gag is not required for the replication of AKV. (ii) A short portion of the leader region diverges in sequence from the closely related Moloney murine leukemia virus and appears to be related to a sequence highly repeated in eucaryotic genomes. (iii) As in Moloney murine leukemia virus, there is a potential RNA secondary structure flanking the amber codon that separates the gag and pol genes. This structure might function as a regulatory protein binding site that controls the relative levels of synthesis of the gag and pol precursors. The nucleotide sequence of the 3' region of AKV is compared with sequences reported previously from both infectious and noninfectious molecular clones of AKV.

Two base changes restore infectivity to a noninfectious molecular clone of Moloney murine leukemia virus (pMLV-1)

The complete nucleotide sequence of a molecular clone of Moloney murine leukemia virus (pMLV-1) has previously been reported (Shinnick et al., Nature [London] 293:543-548, 1981). However, pMLV-1 does not generate infectious virus after transfection into cells (Berns et al., J. Virol. 36:254-263, 1980). The lesion in pMLV-1 has been localized by determining the biological activity of recombinants containing DNA from an infectious clone of Moloney murine leukemia virus (pMLV-48) and pMLV-1. Replacement of a 1.0-kilobase pair region which spans the gag-pol junction of pMLV-1 with the corresponding DNA fragment from the infectious clone restores its infectivity. Nucleotide sequence analysis of this fragment obtained from the infectious clone (pMLV-48) and pMLV-1 reveals two single base pair changes, one in the p30gag gene and the other in the 5' end of the pol gene. The mutation in the pol gene does not affect the production of infectious virus but renders them XC negative, whereas the mutation in the gag gene appears to be lethal. The complete nucleotide sequence of an infectious clone of Moloney murine leukemia virus can now be deduced.