Cell-free translation of frog virus 3 messenger RNAs. Initiation factors from infected cells discriminate between early and late viral mRNAs

Family Iridoviridae: poor viral relations no longer

Members of the family Iridoviridae infect a diverse array of invertebrate and cold-blooded vertebrate hosts and are currently viewed as emerging pathogens of fish and amphibians. Iridovirid replication is unique and involves both nuclear and cytoplasmic compartments, a circularly permuted, terminally redundant genome that, in the case of vertebrate iridoviruses, is also highly methylated, and the efficient shutoff of host macromolecular synthesis. Although initially neglected largely due to the perceived lack of health, environmental, and economic concerns, members of the genus Ranavirus, and the newly recognized genus Megalocytivirus, are rapidly attracting growing interest due to their involvement in amphibian population declines and their adverse impacts on aquaculture. Herein we describe the molecular and genetic basis of viral replication, pathogenesis, and immunity, and discuss viral ecology with reference to members from each of the invertebrate and vertebrate genera.

A preliminary translational map of the frog virus 3 genome

A preliminary map of the frog virus 3 (FV 3) genome was constructed by hybridization-selection of mRNAs to cloned DNA fragments and translation in reticulocyte lysates. FV 3 mRNAs were hybridized to KpnI, HindIII, and SalI restriction fragments representing the entire FV 3 genome. Two different hybridization conditions were employed in order to discriminate between the hybridization of early and late mRNAs. A total of 43 major and 18 minor early genes and nine major and three minor late genes were mapped. A 24 kb region comprised of the KpnI D, H, and E fragments encodes 8 of the 12 late genes that were mapped.

Metabolism of host and viral mRNAs in frog virus 3-infected cells

Treatment of purified frog virus 3 (FV3) with nonionic detergent and high salt released an endoribonucleolytic activity and confirmed earlier findings of a virion-associated endonuclease. This observation, coupled with evidence implicating host and viral message destabilization in herpesvirus and poxvirus biogenesis, raised the question of what role, if any, mRNA degradation plays in FV3 replication. To answer this question, Northern analyses of mock- and virus-infected cells were performed using probes for representative host and viral messages. These studies demonstrated that the steady state level of host messages progressively declined during the course of productive FV3 infection, whereas the steady state level of viral messages was not affected. To determine whether the decline in the steady state level of host mRNA was due to virus-induced degradation or to normal turnover coupled to virus-mediated transcriptional shut-off, actin mRNA levels were examined in mock- and virus-infected cells in the presence and absence of actinomycin D. Under these conditions, actin mRNA levels declined more quickly in actinomycin D-treated, virus-infected cells, than in mock-infected cells incubated in the presence of actinomycin D suggesting that the decline in the steady state level of actin mRNA was due to degradation. However, although it appears as if host message degradation is responsible for virus-mediated translational shut-off, the ability of heat-inactivated FV3 to block cellular translation without destabilizing cellular messages indicates that message degradation is not required for translational inhibition. As noted above, the degradation of early FV3 messages was not involved in controlling the transition from early to late gene expression. Furthermore, the presence of abundant, but nontranslated, early messages late in infection, coupled with the inefficient translation of late messages in vitro supported earlier suggestions that FV3 gene expression is controlled, at least in part, at the translational level. Taken together, these results suggest that FV3 regulates gene expression in a unique manner and may be a good model to examine the mechanics of translational control.

Translational control of equine herpesvirus type 1 gene expression

Translational control mechanisms modulate gene expression in a variety of cellular and viral systems. Using hypertonic conditions to block protein synthesis in vivo, we observed that the synthesis of several major equine herpesvirus type 1 proteins was selectively inhibited. Although sensitivity to hypertonic conditions was graded across a continuum, messages coding for proteins of 203, 130.5, and 31.5 kDa were significantly more resistant to higher salt concentrations in vivo than those coding for polypeptides of 148, 116, and 74 kDa. Similar results were observed in vitro when potassium acetate was used to block initiation. In addition, Northern blot analyses demonstrated that steady-state levels of cellular mRNAs declined beginning at about 6 hr after infection. Taken together, these results indicate that the expression of several major equine herpesvirus type 1 genes was controlled in part at the post-transcriptional level.

Translational efficiency: iridovirus early mRNAs outcompete tobacco mosaic virus message in vitro

Infection with the iridovirus, frog virus 3, results in the rapid inhibition of host cell protein synthesis and is correlated with activation of an eIF-2 kinase. Because phosphorylation of eIF-2 inhibits ternary complex formation and thus reduces the overall level of translation, it has been suggested that frog virus 3 messages escaped translational shut-off by outcompeting host messages for the remaining translational capacity of the cell. In this report, we show that frog virus 3 messages were more translationally competitive than highly efficient tobacco mosaic virus transcripts based on their relative resistance to inhibitors of initiation. This result strengthens the suggestion that the selective translation of frog virus 3 transcripts in virus-infected cells may be a reflection of their enhanced competitiveness.

Frog virus 3-mediated translational shut-off: frog virus 3 messages are translationally more efficient than host and heterologous viral messages under conditions of increased translational stress

Frog virus 3 rapidly and selectively blocks host cell translation while synthesizing more than 60 virus-specific polypeptides. Previous work indicated that virus infection led to activation of a kinase that phosphorylated and, as a consequence, inactivated eIF-2. Although phosphorylation of eIF-2 could explain the rapid decline in host cell translation, it could not explain how viral protein synthesis persisted in the face of host shut-off. To explain this phenomenon, we speculated that viral messages, either as a consequence of their higher translational efficiency or their greater abundance, were able to outcompete host messages for the remaining translational initiation complexes. To test this hypothesis, the relative translational efficiency of three characteristic FV3 messages was measured against that of several model messages. Translational efficiency was determined by monitoring the resistance (and hence the competitiveness) of a given transcript to increasing concentrations of salt in vitro and in vivo. In both rabbit reticulocyte lysates and wheat germ extracts, FV3 messages were more resistant to supra-optimal concentrations of potassium acetate than globin message and three BMV transcripts. In vivo, FV3 polypeptides were synthesized in the presence of salt concentrations that blocked host cell protein synthesis. These results suggest that the selective translation of FV3 messages in virus-infected cells may partly be due to the higher translational efficiency of viral messages. Structural features that contribute to translational efficiency are discussed.

Stimulation of chloramphenicol acetyltransferase mRNA translation by reovirus capsid polypeptide sigma 3 in cotransfected COS cells

The mammalian reovirus S4 gene has been implicated in the serotype-dependent inhibition of host cell protein synthesis during viral replication in mouse L cells. To examine the effect(s) of this gene on transcription or translation or both, a DNA copy of the serotype 3 S4 gene was inserted into a eucaryotic expression vector. Cotransfection of COS cells with plasmids containing S4 and the reporter gene, chloramphenicol acetyltransferase (CAT), resulted in a marked stimulation of CAT expression, predominantly at the level of translation. The significance of these findings is discussed in relation to the double-stranded-RNA-binding activity of the S4 gene product, polypeptide sigma 3.

Organization of RNA transcripts from a 7.8-kb region of the frog virus 3 genome

The detailed organization of the RNAs transcribed from a region of the FV 3 genome (Sa/I-F fragment and adjacent sequences) has been determined. The information was derived from the cell-free translation of hybrid-selected RNA to locate the genes encoding specific polypeptides, RNA filter hybridization to size the transcripts, and S1 nuclease mapping to locate the 5'- and 3'-ends of the RNAs on the genome. Three genes are contiguous and are transcribed from the same strand: two immediate early genes encoding transcripts of about 1.3 kb that directed the in vitro synthesis of 42K and 46K polypeptides, separated by the late gene encoding the major capsid protein (48K). At an advanced stage in infection, transcripts derived from the immediate early genes are also present. A set of RNAs with different 5'-ends ranging from 1.7 to 0.58 kb is produced from the p46 gene region whereas RNAs, 0.98 and 0.6 kb in size, complementary to the 5'-end of the p42 message, are synthesized. This gene cluster is located between two genes transcribed in the opposite direction from the rightward-reading strand: a late gene whose message is 0.5 kb in size and encodes a 15K polypeptide and a gene transcribed at immediate early and late times of infection which encodes a protein of 70 kDa. The 5'-end of the late RNA maps downstream of the 5'-end of the early one, their sizes being 1.85 and 2 kb, respectively, but both of them can be translated in vitro into a 70K polypeptide. These observations suggest that transcription is not regulated by the organization of the genes; they suggest rather that specific DNA sequences are responsible for the promotion of immediate early and late transcriptions.

Molecular cloning and physical and translational mapping of the frog virus 3 genome

A library of cloned fragments representing nearly the entire frog virus 3 (FV 3) genome (99.65%) has been constituted. Individual plasmid recombinants, labeled by nick-translation, were hybridized to Southern blots of genomic FV 3 DNA fragments obtained with XbaI, HindIII, SmaI, and SalI. From these results physical maps were generated and the distribution of restriction sites in the genome was established by double digestion of the fragments. A preliminary translational map was likewise developed. The viral messages were selected by hybridization to the recombinant DNAs immobilized on nitrocellulose filters and were translated in the reticulocyte cell-free system. About 30 polypeptides were detected among the translation products of RNA synthesized in the presence of cycloheximide. It appears that these genes are not clustered but in several cases more than one polypeptide is encoded by a given fragment. The 15 new polypeptide obtained by translation of late mRNAs derive from genes located on one-half of the genome.

Thermosensitivity of frog virus 3 genome expression: defect in early transcription

The influence of temperature on the transcription of the frog virus 3 genome was studied in CHO cells infected both at 29 and at 37 degrees, the nonpermissive temperature for virus multiplication. It was definitely established that late genes were not transcribed at 37 degrees. Although immediate early genes were expressed at 37 degrees, their transcription was altered but there was no sequestration of mRNAs in the nucleus which could impair their translation; these viral mRNAs were also efficiently translated in vitro. These results indicate that an immediate early viral protein involved in the transcription of delayed early genes is likely to be thermosensitive. Furthermore, one event taking place at the very beginning of the infection, possibly related to the activity of a viral structural component, facilitates the transcription of immediate early genes at 29 degrees and this step is partially impaired at 37 degrees.

Restriction of frog virus 3 polypeptide synthesis to immediate early and delayed early species by supraoptimal temperatures

Multiplication of frog virus 3 (FV 3) occurs in mammalian cells provided they are incubated at temperatures lower than 33 degrees. The expression of the viral genome at supraoptimal temperatures was followed by analyzing the polypeptides produced in CHO-infected cells and comparing with those obtained under restrictive conditions provoked by amino acid analogs or metabolic inhibitors. Late polypeptides were not detected at 33 degrees and the number of delayed early species decreased gradually with increasing temperatures consequently the synthesis of all delayed early proteins was not turned on in response to a unique event. At 37 degrees the synthesis was limited to the immediate early species, i.e., the proteins synthesized after cycloheximide reversal. Temperature shift experiments suggested that delayed early genes remained untranscribed at 37 degrees. Thus, incubation of FV 3-infected mammalian cells at 37 degrees provides a unique way of limiting viral synthesis to immediate early proteins without the side-effect provoked by inhibitors.

Temperature-sensitive mutants of frog virus 3: biochemical and genetic characterization

Nineteen frog virus 3 temperature-sensitive mutants were isolated after mutagenesis with nitrosoguanidine and assayed for viral DNA, RNA, and protein synthesis, as well as assembly site formation at permissive (25 degrees C) and nonpermissive (30 degrees C) temperatures. In addition, mutants were characterized for complementation by both quantitative and qualitative assays. Based on the genetic and biochemical data, the 19 mutants, along with 9 mutants isolated earlier, were ordered into four phenotypic classes which define defects in virion morphogenesis (class I), late mRNA synthesis (class II), viral assembly site formation (class III), and viral DNA synthesis (class IV). In addition, we used two-factor crosses to order 11 mutants, comprising 7 complementation groups, onto a linkage map spanning 77 recombination units.

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.

Translational discrimination between the four RNAs of alfalfa mosaic virus. A quantitative evaluation

In an attempt to relate the translational characteristics of alfalfa mosaic virus (A1MV) RNAs to their structure [Ravelonandro et al. (1983) Nucleic Acids Res. 11, 2815-2826; Gehrke et al. (1983) Biochemistry 22, 5157-5164] we measured the relative affinities (discrimination ratios) of these RNAs for the initiation complex, in the wheat germ extract and in the nuclease-treated reticulocyte lysate, using a competition method designed by Brendler et al. [(1981) J. Biol. Chem. 256, 11747-11754]. As a prerequisite of this study we ascertained that the molecular mass distribution of the translation products was independent of RNA concentration in both translation systems. In the wheat germ extract the discrimination ratios are very similar for two strains of A1MV (S and B) which differ mainly by the presence (strain S) or absence (strain B) of a stable 5'-proximal hairpin. Hence this structure has no bearing on discrimination. Taking the affinity of RNA 3 as reference, the following orders of magnitude are found for the affinities of the different RNAs in the wheat germ: RNA 3, 1.0; RNA 1, 10; RNA 2, 60; RNA 4, 150. In the reticulocyte lysate the discrimination ratios are not significantly different from the wheat germ. Thus it seems that the mechanism of discrimination is essentially the same in the two translation systems, despite a difference in rate-limitation.