Isolation and preliminary characterization of temperature-sensitive mutants of poliovirus type 1

Animal and human RNA viruses: genetic variability and ability to overcome vaccines

RNA viruses, in general, exhibit high mutation rates; this is mainly due to the low fidelity displayed by the RNA-dependent polymerases required for their replication that lack the proofreading machinery to correct misincorporated nucleotides and produce high mutation rates. This lack of replication fidelity, together with the fact that RNA viruses can undergo spontaneous mutations, results in genetic variants displaying different viral morphogenesis, as well as variation on their surface glycoproteins that affect viral antigenicity. This diverse viral population, routinely containing a variety of mutants, is known as a viral 'quasispecies'. The mutability of their virions allows for fast evolution of RNA viruses that develop antiviral resistance and overcome vaccines much more rapidly than DNA viruses. This also translates into the fact that pathogenic RNA viruses, that cause many diseases and deaths in humans, represent the major viral group involved in zoonotic disease transmission, and are responsible for worldwide pandemics.

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.

Use of temperature-sensitive mutants to study the morphogenesis of poliovirus

Three temperature-sensitive (ts) mutants of poliovirus (type 1 Mahoney) were isolated after nitrous acid treatment and characterized as phenotypically RNA+. When cells were infected at 37 degrees with two of the three RNA+ ts mutants (ts109 and ts739), reduced levels of 14 S particles were synthesized. One RNA+ mutant (ts520) synthesized significant amounts of viral 14 S particle subunits. All of the mutants synthesized reduced amounts of procapsids and virions at 37 degrees. At 39.5 degrees, with all three ts mutants, the production of all virus-related particles in infected cells was markedly suppressed. Isoelectric focusing of the viral-related particles produced at 37 degrees by the ts mutants and electrophoretic analysis of their structural polypeptides revealed the following: (i) ts739 synthesized an altered VP0 polypeptide and produced 14 S particles with an altered isoelectric point; (ii) ts109 produced 14 S particles with a normal pI but containing what appeared to be an altered VP1; (iii) ts520 produced normal 14 S particles as demonstrated by their pI, the electrophoretic behavior of their constituent structural polypeptides in SDS-PAGE, their ability to self-assemble, and their ability to form procapsid-like structures when incubated in extracts from wild-type (wt) virus-infected cells. However, ts520-infected cells contained few, if any, procapsids and extracts made therefrom were unable to assemble ts520 or wt 14 S particles into detectable amounts of pI 6.8 empty capsids. These and other findings are consistent with ts739 (and probably ts109) possessing an altered structural protein and ts520 being mutant in its morphopoietic factor.

Preliminary characterization of coxsackievirus B3 temperature-sensitive mutants

Prototype temperature-sensitive (ts) mutants of a coxsackievirus B3 parent virus capable of replication to similar levels at 34 or 39.5 degrees C were examined for the nature of the temperature-sensitive event restricting replication in HeLa cells at 39.5 degrees C. The ts mutant prototypes represented three different non-overlapping complementation groups. The ts1 mutant (complementation group III) synthesized less than 1% of the infectious genomic RNA synthesized by the coxsackievirus B3 parent virus at 39.5 degrees C and was designated an RNA- mutant. Agarose gel analysis of glyoxal-treated RNA from cells inoculated with ts1 virus revealed that cell RNA synthesis continued in the presence of synthesis of the small amount of viral RNA. This mutant was comparatively ineffective in inducing cell cytopathology and in directing synthesis of viral polypeptides, likely due to the paucity of nascent genomes for translation. The ts5 mutant (complementation group II) directed synthesis of appreciable quantities of both viral genomes (RNA+) and capsid polypeptides; however, assembly of these products into virions occurred at a low frequency, and virions assembled at 39.5 degrees C were highly unstable at that temperature. Shift-down experiments with ts5-inoculated cells showed that capsid precursor materials synthesized at 39.5 degrees C can, after shift to 34 degrees C, be incorporated into ts5 virions. We suggest that the temperature-sensitive defect in this prototype is in the synthesis of one of the capsid polypeptides that cannot renature into the correct configuration required for stability in the capsid at 39.5 degrees C. The ts11 mutant (complementation group I) also synthesized appreciable amounts of viral genomes (RNA+) and viral polypeptides at 39.5 degrees C. Assembly of ts11 virions at 39.5 degrees C occurred at a low frequency, and the stability of these virions at 39.5 degrees C was similar to that of the parent coxsackievirus B3 virions. The temperature-sensitive defect in the ts11 prototype is apparently in assembly. The differences in biochemical properties of the three prototype ts mutants at temperatures above 34 degrees C may ultimately offer insight into the differences in pathogenicity observed in neonatal mice for the three prototype ts mutants.