Cell-free synthesis of Rauscher murine leukemia virus "gag" and "env" gene products from separate cellular mRNA species

Characterization of an internal ribosomal entry segment in the 5' leader of murine leukemia virus env RNA

The 5' untranslated region, also called the leader, of oncoretroviruses and lentiviruses is long and is formed of several structured domains critically important in virus replication. The 5' leader of murine leukemia virus (MLV) RNA contains an internal ribosomal entry segment (IRES) which promotes synthesis of Gag and glyco-Gag polyprotein precursors. In the present study we investigated the translational features of the 5' leader of MLV subgenomic RNA (env RNA) encoding the Env polyprotein precursor. When the env leader was inserted between two genes, such as lacZ and the neomycin resistance cassette, in a dicistronic vector, it allowed IRES-dependent translation of the 3' cistron in the rabbit reticulocyte lysate system and in murine cells. The drug rapamycin and the foot-and-mouth disease virus L protease, known to inhibit cap-dependent translation, caused an enhancement of the translation driven by the env leader sequence, consistent with an IRES activity promoting Env expression. Analysis of several deletion mutants led us to localize the minimal env IRES between the splice junction and the env AUG start codon.

Isolation and characterization of a retrovirus from the fish genus Xiphophorus

A cell line (BsT) established from neoplastic embryonal tissues of the platyfish (Xiphophorus maculatus) released spontaneously retrovirus-like particles. The particles have a buoyant density of 1.16 g/ml, a mean diameter of 100 nm and the morphology of immature retroviruses. The particle-associated proteins p70, p65, and p28 react with an antiserum directed against the major internal feline leukemia virus structural protein p27. The particles are associated with a reverse transcriptase. The purified enzyme has a molecular weight of about 70 kDa and prefers the template primers poly(rA):oligo(dT), poly(dC):oligo(dG), and poly(rC):oligo(dG) in the presence of Mn2+. The enzyme activity is inhibited by antibodies directed against the reverse transcriptase of feline leukemia virus and simian sarcoma virus. The particles contain a ribonucleic acid of about 70 S. In an endogenous reverse transcriptase reaction nucleic acids in the range of 0.2 to 0.4 kb were synthesized. In Northern blots with these nucleic acids as probe, three transcripts of about 8.5, 4.2, and 1.5 kb were detected in BsT cells. Southern blot analysis with the same probe demonstrates related sequences in the DNA of BsT cells and the platyfish and swordtail (Xiphophorus helleri). Hybridization experiments with the LTR-gag region of the feline leukemia virus show homologous sequences in the Xiphophorus genome.

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.

Translation of messenger RNA from rat epididymis and identification of poly(A)RNA coding for acidic epididymal glycoprotein

Different proteins are secreted by the various regions of rat epididymis. We have examined the messenger RNA dependence of this varied gene expression by cell-free translation of poly(A)RNA extracts from initial segment, caput plus corpus, and cauda. Labeled translation products were analyzed by polyacrylamide gel electrophoresis under denaturing conditions. Poly(A)RNA from initial segment coded for several unique bands of labeled protein, including a 23,000 MW protein that may correspond to an initial segment protein reported previously to be regulated by testicular fluid factors. Messenger RNA encoding 20,000 MW protein believed to be alpha-lactalbumin, was most abundant in caput but also present in initial segment. Acidic epididymal glycoprotein (AEG) was identified previously by immunoperoxidase staining in epithelial cells of caput, corpus, and cauda. AEG was not readily identified on electrophoresis of total translated proteins, but when concentrated by immunoprecipitation with purified AEG antibody prior to electrophoresis, AEG appeared in both caput plus corpus, and in cauda poly(A)RNA translations.

In vitro synthesis and glycosylation of androgen-dependent secretory proteins of rat dorsal prostate and coagulating gland

The two major androgen-dependent secretory proteins of rat dorsal prostate and coagulating gland, DP I and DP II, were compared by in vitro translation of total poly(A)RNA and by pulse-chase techniques by means of [35S]methionine and tissue minces of coagulating gland. DP I is a major in vitro translation product of isolated poly(A)RNA, whereas DP II is undetectable in a mouse embryo fibroblast cell-free system where glycosylation does not occur. DP I is synthesized within 20 min in minces of coagulating gland incubated in the presence of [35S]methionine and is secreted in 40 min. DP II is detectable in the medium only after 8 hr of labeling. Inhibition of asparagine-linked protein glycosylation with tunicamycin (10 micrograms/ml) blocked the synthesis and secretion of DP II with an apparent increase in DP I secretion. Inhibition of DP II synthesis by monensin implicates the Golgi in the processing of DP II oligosaccharides. The data are consistent with the proposal that DP I enters a pathway of rapid secretion that is enhanced by the absence of core glycosylation, whereas DP II follows a slow pathway through the Golgi that involves extensive glycosylation.

Provirus of M7 baboon endogenous virus: nucleotide sequence of the gag-pol region

A 3,023-base nucleotide sequence of the M7 baboon endogenous virus genome, spanning the 5' noncoding region as well as the entire gag gene and part of the pol gene, is reported. Within the 562-base 5' noncoding region, a 21-base sequence complementary to the OH terminus of tRNApro is located immediately downstream from the long terminal repeat. Amino acid sequences were deduced from the 1,596 nucleotides comprising the gag gene, and the four structural gag polypeptides, p12, p15, p30, and p10, appeared to be coded contiguously. Only one termination codon interrupted the M7 gag and pol genes. The data suggest that 55 additional amino acids may be attached to the NH2 terminus of the gag precursor protein. However, such a sequence was not detected in virions or in virus-infected cells. With the exception of the p15 region, nucleotide and amino acid sequences of the gag and pol regions of M7 virus exhibited strong homologies to those of Moloney leukemia virus.

Purification and translation of murine mammary tumor virus mRNA's

We have studied the functions of the intracellular RNAs of mouse mammary tumor virus (MMTV) by purification and translation in vitro. Two major size classes of MMTV RNA, 35S and 24S RNA, were isolated from MMTV-infected rat (XC) cells and cultured mammary tumor cells by preparative hybridization of whole cell or polyadenylated RNA to cloned MMTV DNA covalently bound to chemically activated paper disks (diazobenzyloxymethyl paper). Genomic-length (35S) RNA was prepared free of 24S RNA by rate zonal sedimentation in sucrose gradients. Experiments using [3H]uridine-labeled cellular RNA indicated that the preparative annealing method was highly specific and capable of effecting a 300-fold enrichment for viral RNA; the recovered RNA appeared to be intact under denaturing conditions and directed synthesis of full-length gag and env polypeptides in vitro. The products of in vitro translation were identified by gel mobility, immunoprecipitation tests with antisera against gag and env products, and partial digestion with Staphylococcus V8 protease. The 35S RNA species directed synthesis of several gag-related polypeptides, including three previously reported in extracts of infected cells; 24S RNA directed synthesis of two polypeptides closely related to env proteins from infected cells. Therefore, 35S RNA includes mRNA's for gag and gag-pol, whereas 24S RNA is the mRNA for env. These results help establish the position of env on the physical map of the MMTV genome and bear upon the coding potential of the genome.

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

The effect of suppressor tRNA's on the cell-free translation of several leukemia and sarcoma virus RNAs was examined. Yeast amber suppressor tRNA (amber tRNA) enhanced the synthesis of the Rauscher murine leukemia virus and clone 1 Moloney murine leukemia virus Pr200(gag-pol) polypeptides by 10- to 45-fold, but at the same time depressed the synthesis of Rauscher murine leukemia virus Pr65(gag) and Moloney murine leukemia virus Pr63(gag). Under suppressor-minus conditions, Moloney murine leukemia virus Pr70(gag) was present as a closely spaced doublet. Amber tRNA stimulated the synthesis of the "upper" Moloney murine leukemia virus Pr70(gag) polypeptide. Yeast ochre suppressor tRNA appeared to be ineffective. Quantitative analyses of the kinetics of viral precursor polypeptide accumulation in the presence of amber tRNA showed that during linear protein synthesis, the increase in accumulated Moloney murine leukemia virus Pr200(gag-pol) coincided closely with the molar loss of Pr63(gag). Enhancement of Pr200(gag-pol) and Pr70(gag) by amber tRNA persisted in the presence of pactamycin, a drug which blocks the initiation of protein synthesis, thus arguing for the addition of amino acids to the C terminus of Pr63(gag) as the mechanism behind the amber tRNA effect. Moloney murine sarcoma virus 124 30S RNA was translated into four major polypeptides, Pr63(gag), P42, P38, and P23. In the presence of amber tRNA, a new polypeptide, Pr67(gag), appeared, whereas Pr63(gag) synthesis was decreased. Quantitative estimates indicated that for every 1 mol of Pr67(gag) which appeared, 1 mol of Pr63(gag) was lost.