The time course of synthesis of poliovirus RNA

Joys and surprises of a career studying eukaryotic gene expression

In this Reflections, I review a few early and very lucky events that gave me a running start for the rest of a long and wonderfully enjoyable career. For the main part, a discussion is provided of what I recall as the main illuminating results that my many dozens of students and postdoctoral fellows (approximately 140 in all) provided to our biochemical/molecular biological world.

Translating ribosomes inhibit poliovirus negative-strand RNA synthesis

Poliovirus has a single-stranded RNA genome of positive polarity that serves two essential functions at the start of the viral replication cycle in infected cells. First, it is translated to synthesize viral proteins and, second, it is copied by the viral polymerase to synthesize negative-strand RNA. We investigated these two reactions by using HeLa S10 in vitro translation-RNA replication reactions. Preinitiation RNA replication complexes were isolated from these reactions and then used to measure the sequential synthesis of negative- and positive-strand RNAs in the presence of different protein synthesis inhibitors. Puromycin was found to stimulate RNA replication overall. In contrast, RNA replication was inhibited by diphtheria toxin, cycloheximide, anisomycin, and ricin A chain. Dose-response experiments showed that precisely the same concentration of a specific drug was required to inhibit protein synthesis and to either stimulate or inhibit RNA replication. This suggested that the ability of these drugs to affect RNA replication was linked to their ability to alter the normal clearance of translating ribosomes from the input viral RNA. Consistent with this idea was the finding that the protein synthesis inhibitors had no measurable effect on positive-strand synthesis in normal RNA replication complexes. In marked contrast, negative-strand synthesis was stimulated by puromycin and was inhibited by cycloheximide. Puromycin causes polypeptide chain termination and induces the dissociation of polyribosomes from mRNA. Cycloheximide and other inhibitors of polypeptide chain elongation "freeze" ribosomes on mRNA and prevent the normal clearance of ribosomes from viral RNA templates. Therefore, it appears that the poliovirus polymerase was not able to dislodge translating ribosomes from viral RNA templates and mediate the switch from translation to negative-strand synthesis. Instead, the initiation of negative-strand synthesis appears to be coordinately regulated with the natural clearance of translating ribosomes to avoid the dilemma of ribosome-polymerase collisions.

RNA-protein interactions directed by the 3' end of human rhinovirus genomic RNA

The replication of a picornavirus genomic RNA is a template-specific process involving the recognition of viral RNAs as target replication templates for the membrane-bound viral replication initiation complex. The virus-encoded RNA-dependent RNA polymerase, 3Dpol, is a major component of the replication complex; however, when supplied with a primed template, 3Dpol is capable of copying polyadenylated RNAs which are not of viral origin. Therefore, there must be some other molecular mechanism to direct the specific assembly of the replication initiation complex at the 3' end of viral genomic RNAs, presumably involving cis-acting binding determinants within the 3' noncoding region (3' NCR). This report describes the use of an in vitro UV cross-linking assay to identify proteins which interact with the 3' NCR of human rhinovirus 14 RNA. A cellular protein(s) was identified in cytoplasmic extracts from human rhinovirus 14-infected cells which had a marked binding preference for RNAs containing the rhinovirus 3' NCR sequence. This protein(s) showed reduced cross-linking efficiency for a 3' NCR with an engineered deletion. Virus recovered from RNA transfections with in vitro transcribed RNA containing the same 3' NCR deletion demonstrated a defective replication phenotype in vivo. Cross-linking experiments with RNAs containing the poliovirus 3' NCR and cytoplasmic extracts from poliovirus-infected cells produced an RNA-protein complex with indistinguishable electrophoretic properties, suggesting that the appearance of the cellular protein(s) may be a common phenomenon of picornavirus infection. We suggest that the observed cellular protein(s) is sequestered or modified as a result of rhinovirus or poliovirus infection and is utilized in viral RNA replication, perhaps by binding to the 3' NCR as a prerequisite for replication complex assembly at the 3' end of the viral genomic RNA.

In vitro synthesis and assembly of picornaviral capsid intermediate structures

Cell-free translation of encephalomyocarditis RNA in extracts of rabbit reticulocytes results in the synthesis of viral proteins indistinguishable from those produced during virus infection of cells. The viral capsid proteins are produced in an active form capable of assembly into viral capsid intermediate structures. Protomers (5S), pentamers (14S), and shell-like structures (75 to 85S) can be detected after prolonged incubation in the extracts. Proteolytic cleavage of capsid precursor proteins appears to be a prerequisite for assembly, in apparent contrast to cell-associated assembly. Assembly of pentamers is also preceded by conversion of protein epsilon 1 to epsilon in a step which may reflect an amino-terminal blocking reaction.

Dietary hepatic cholesterol elevation: effects on coxsackievirus B infection and inflammation

Mice made hypercholesterolemic (HC) by diet are highly susceptible to coxsackievirus (CV) B5, whereas normal adult animals remain resistant. In attempting to define those dietary-induced physiological changes which contribute to altered resistance, a strong association between accumulation of intrahepatic cholesterol and increased CV B5-induced mortality was demonstrated, with maximum susceptibility to CV coinciding with a 2.5-fold increase in the ratio of hepatic cholesterol to protein. This metabolic imbalance was associated with a lower clearance rate of CV from the blood and liver of C57BL/6 mice, although virus-specific neutralizing antibody production was unaltered. In addition to CV, HC mice were more susceptible to an intravenous inoculation of Listeria monocytogenes in comparison to controls. The macrophage stimulant Corynebacterium parvum failed to increase resistance of HC mice to a high dose of CV B4 and L. monocytogenes and failed to induce the hepatomegaly, splenomegaly, and cellular infiltrate seen in the liver and spleen of normal animals. Furthermore, the peritoneal monocytic infiltrate induced by thioglycolate in normal animals was absent in HC mice. Results from these experiments suggest that decreased resistance to CV in the HC host is attributed to a defect in the nonspecific immune responses of macrophages and monocytes which are of primary importance in resistance to this virus and other infectious agents.

Chemically-induced temperature sensitive mutants of dengue virus type 2. I. Isolation and partial characterization

Temperature sensitive (ts) mutants of dengue virus type 2 (DEN-2, TH-36 isolate) were induced by replication in primary hamster kidney cells treated with 5-azacytidine. Seven ts mutants were obtained from 138 clones isolated by an immunofluorescent cloning technique. Of these 7 ts mutants, 5 were sufficiently stable to permit partial characterization. Complementation was detected at very low but statistically significant levels between some ts mutants at 40 degrees C. Viral double-stranded RNA production was evaluated in LLC-MK2 cells at 30 degrees and 40 degrees C by micro-quantitative complement fixation. The results of complementation tests and RNA production tests indicated that the 4 of 5 stable ts mutants constitute 3 separate complementation groups (2 RNA+ and 1 RNA-groups), while a fifth ts mutant was RNA- but non-complementable. The data presented here indicate that a genetic system can be developed without employing traditional plaque or cytopathology methods. Further, the 5 DEN-2 ts mutants are believed to represent the only set of complementation-positive flavivirus mutants so far isolated.

The preparation of virus-infected cells for use in immunofluorescence techniques

This paper describes the preparation of virus-infected cells for the detection of serum antibodies by the use of immunofluorescence techniques. The variations in virus growth in cells in relation to the length of incubation of cell monolayers and polio virus using two different fluorescein isothiocynate conjugates are discussed. It is shown that there is a critical time after virus infection when a definite positive immunofluorescent reaction is given. Furthermore, a method is described enabling the clinical pathologist to prepare infected cells with the use of an automatic tube inoculator at any time during 24 hours.

Studies of the activity of diphtheria toxin. I. Poliovirus replication in intoxicated HeLa cells

It has been demonstrated that a saturating dose of diphtheria toxin produced a 90% inhibition of polio-virus replication in HeLa cells. This inhibition was reflected in infectious viral RNA synthesis and in mature virus production. Toxin had no direct effect on virus particles or I-RNA, and poliovirus adsorption and eclipse appeared to be carried out normally in intoxicated cells. When toxin was given at various time intervals after infection, the amount of inhibition depended on the time of toxin addition. Toxin given before or immediately after infection gave maximum inhibition, while toxin given several hours after infection had little effect. The data suggest that toxin inhibits viral replication through its effect on protein synthesis. It is likely that a critical step in the viral replication cycle, the production of poliovirus-induced RNA polymerase, is inhibited, and possibly the synthesis of capsid protein. Ammonium salts and the aliphatic amines, glycamine and prolamine, prevented the inhibition of viral replication by toxin. The kinetics of the protective action of ammonium chloride and diphtheria antitoxin are remarkably similar.

Reoviruses II. Structure and Composition of the Virion

Mayor, Heather D. (Baylor University College of Medicine, Houston, Tex.), Richard M. Jamison, Liane E. Jordan, and M. Van Mitchell . Reoviruses. II. Structure and composition of the virion. J. Bacteriol. 89: 1548–1556. 1965.—Reovirus type 1 has been grown in green monkey kidney cells and harvested 24 hr after infection (“early” virus) and 96 hr after infection (“late” virus). A number of biological parameters have been determined on purified preparations of both “early” and “late” harvests of reovirus. There were no significant differences in values obtained for the molecular weight, RNA content, and buoyant density of virions prepared from early or late harvests. The size of the capsids and their morphology were also identical. Late harvests of reovirus were particularly rich in empty viral capsids (density, 1.28 in cesium chloride), and a significant number of empty inner capsid shells were routinely found. These shells could be prepared readily by controlled digestion of complete virus particles with trypsin. The inner shell appears to be composed of subunits packed with icosahedral symmetry to form a 45-mμ foundation on which the outer 92-subunit capsid is assembled. The inner shell is somewhat reminiscent in size and morphology of the capsid of papovaviruses. The fact that it can exist as a discrete entity has prompted us to propose some modifications to the current models for the reovirus capsid.