Mechanism of vesicular stomatitis virus mRNA decay

New mRNAs are preferentially translated during vesicular stomatitis virus infection

During vesicular stomatitis virus (VSV) infection, host protein synthesis is inhibited, while synthesis of viral proteins increases. VSV infection causes inhibition of host transcription and RNA transport. Therefore, most host mRNAs in the cytoplasm of infected cells were synthesized before infection. However, viral mRNAs are synthesized throughout infection and are newer than preexisting host mRNAs. To determine if the timing of appearance of mRNAs in the cytoplasm affected their translation during VSV infection, we transfected reporter mRNAs into cells at various times relative to the time of infection and measured their rate of translation in mock- and VSV-infected cells. We found that translation of mRNAs transfected during infection was not inhibited but that translation of mRNAs transfected prior to infection was inhibited during VSV infection. Based on these data, we conclude that the timing of viral mRNA appearance in the cytoplasm is responsible, at least in part, for the preferential translation of VSV mRNAs. A time course measuring translation efficiencies of viral and host mRNAs showed that the translation efficiencies of viral mRNAs increased between 4 and 8 h postinfection, while translation efficiencies of host mRNAs decreased. The increased translation efficiency of viral mRNAs occurred in cells infected with an M protein mutant virus that is defective in host shutoff, demonstrating that the enhanced translation of viral mRNA is genetically separable from inhibition of translation of host mRNA.

Model-based design of growth-attenuated viruses

Live-virus vaccines activate both humoral and cell-mediated immunity, require only a single boosting, and generally provide longer immune protection than killed or subunit vaccines. However, growth of live-virus vaccines must be attenuated to minimize their potential pathogenic effects, and mechanisms of attenuation by conventional serial-transfer viral adaptation are not well-understood. New methods of attenuation based on rational engineering of viral genomes may offer a potentially greater control if one can link defined genetic modifications to changes in virus growth. To begin to establish such links between genotype and growth phenotype, we developed a computer model for the intracellular growth of vesicular stomatitis virus (VSV), a well-studied, nonsegmented, negative-stranded RNA virus. Our model incorporated established regulatory mechanisms of VSV while integrating key wild-type infection steps: hijacking of host resources, transcription, translation, and replication, followed by assembly and release of progeny VSV particles. Generalization of the wild-type model to allow for genome rearrangements matched the experimentally observed attenuation ranking for recombinant VSV strains that altered the genome position of their nucleocapsid gene. Finally, our simulations captured previously reported experimental results showing how altering the positions of other VSV genes has the potential to attenuate the VSV growth while overexpressing the immunogenic VSV surface glycoprotein. Such models will facilitate the engineering of new live-virus vaccines by linking genomic manipulations to controlled changes in virus gene-expression and growth.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

The stability of mRNA for eucaryotic elongation factor Tu in Friend erythroleukemia cells varies with growth rate

The decay rates of eucaryotic elongation factor Tu (eEF-Tu) mRNA and eucaryotic initiation factor 4A (eIF-4A) mRNA in Friend erythroleukemia (FEL) cells were determined under several different growth conditions. In FEL cells which were no longer actively dividing (stationary phase), eEF-Tu mRNA was found to be rather stable, with a t1/2 of about 24 h. In rapidly growing FEL cells eEF-Tu mRNA was considerably less stable, with a t1/2 of about 9 h. In both cases a single rate of mRNA decay was observed. However, when stationary-phase cells resumed growth after treatment with fresh medium, we observed that eEF-Tu mRNA decay followed a biphasic process. The faster of the two decay rates involved approximately 50% of the eEF-Tu mRNA and had a t1/2 of about 1 h. The decay rates for eIF-4A (t1/2 = 2 h) and total poly(A)+ RNA (t1/2 = 3 h) were unaffected by changes in growth conditions. The t1/2 for polysomal eEF-Tu mRNA was found to be about 8 h when stationary FEL cells were treated with fresh medium. Previous work in this laboratory has shown (T. R. Rao and L. I. Slobin, Mol. Cell. Biol. 7:687-697, 1987) that when FEL cells are allowed to grow to stationary phase, approximately 60% of the mRNA for eEF-Tu is found in a nontranslating postpolysomal messenger ribonucleoprotein (mRNP) particle. eEF-Tu mRNP was rapidly cleared from stationary cells after treatment with fresh medium. The data presented in this report indicate that the stability of eEF-Tu mRNP is rapidly altered and the particle is targeted for degradation when stationary FEL cells resume growth.

Role of protein synthesis in decay and accumulation of mRNA during spore germination in the cellular slime mold Dictyostelium discoideum

Spore germination in Dictyostelium discoideum is a particularly suitable model for studying the regulation of gene expression, since developmentally regulated changes in both protein and mRNA synthesis occur during the transition from dormant spore to amoeba. The previous isolation of three cDNA clones specific for mRNA developmentally regulated during spore germination allowed for the quantitation of the specific mRNAs during this process. The three mRNAs specific to clones pLK109, pLK229, and pRK270 have half-lives much shorter (minutes) than those of constitutive mRNAs (hours). Using spore germination as a model, we studied the roles of ribosome-mRNA interactions and protein synthesis in mRNA degradation by using antibiotics that inhibit specific reactions in protein biosynthesis. Cycloheximide inhibits the elongation step of protein synthesis. Polysomes accumulate in inhibited cells because ribosomes do not terminate normally and new ribosomes enter the polysome, eventually saturating the mRNA. Pactamycin inhibits initiation, and consequently polysomes break down in the presence of this drug. Under this condition, the mRNA is essentially free of ribosomes. pLK109, pLK229, and pRK270 mRNAs were stabilized in the presence of cycloheximide, but pactamycin had no effect on their normal decay. Since it seems likely that stability of mRNA reflects the availability of sites for inactivation by nucleases, it follows that in the presence of cycloheximide, these sites are protected, presumably by occupancy by ribosomes. No ribosomes are bound to mRNA in the presence of pactamycin, and therefore mRNA degrades at about the normal rate. The data further indicate that a labile protein is probably not involved in mRNA decay or stabilization, since protein synthesis is inhibited equally by both antibiotics. We conclude that it may be important to use more than one type of protein synthesis inhibitor to evaluate whether protein synthesis is required for mRNA decay. The effect of protein synthesis inhibition on mRNA synthesis and accumulation was also studied. mRNA synthesis continues in the presence of inhibitors, albeit at a diminished rate relative to that of the uninhibited control.

Role of protein synthesis in decay and accumulation of mRNA during spore germination in the cellular slime mold Dictyostelium discoideum

Spore germination in Dictyostelium discoideum is a particularly suitable model for studying the regulation of gene expression, since developmentally regulated changes in both protein and mRNA synthesis occur during the transition from dormant spore to amoeba. The previous isolation of three cDNA clones specific for mRNA developmentally regulated during spore germination allowed for the quantitation of the specific mRNAs during this process. The three mRNAs specific to clones pLK109, pLK229, and pRK270 have half-lives much shorter (minutes) than those of constitutive mRNAs (hours). Using spore germination as a model, we studied the roles of ribosome-mRNA interactions and protein synthesis in mRNA degradation by using antibiotics that inhibit specific reactions in protein biosynthesis. Cycloheximide inhibits the elongation step of protein synthesis. Polysomes accumulate in inhibited cells because ribosomes do not terminate normally and new ribosomes enter the polysome, eventually saturating the mRNA. Pactamycin inhibits initiation, and consequently polysomes break down in the presence of this drug. Under this condition, the mRNA is essentially free of ribosomes. pLK109, pLK229, and pRK270 mRNAs were stabilized in the presence of cycloheximide, but pactamycin had no effect on their normal decay. Since it seems likely that stability of mRNA reflects the availability of sites for inactivation by nucleases, it follows that in the presence of cycloheximide, these sites are protected, presumably by occupancy by ribosomes. No ribosomes are bound to mRNA in the presence of pactamycin, and therefore mRNA degrades at about the normal rate. The data further indicate that a labile protein is probably not involved in mRNA decay or stabilization, since protein synthesis is inhibited equally by both antibiotics. We conclude that it may be important to use more than one type of protein synthesis inhibitor to evaluate whether protein synthesis is required for mRNA decay. The effect of protein synthesis inhibition on mRNA synthesis and accumulation was also studied. mRNA synthesis continues in the presence of inhibitors, albeit at a diminished rate relative to that of the uninhibited control.