The second origin of the quasispecies concept was experimental. It had long been suspected that RNA viruses were genetically unstable, as suggested by the abundance of mutants in viral stocks and by the difficulty of maintaining some mutants (conditional lethal, plaque-type) free of wild-type revertants. The first evidence that an RNA virus depicted features of quasispecies was obtained by Weissmann and associates in Zürich working with bacteriophage Qß. Upon replication in its host Escherichia coli, a clone of bacteriophage Qß generated error copies with high frequency. Quantification of the reversion of a site-specific mutant, and of its growth rate relative to the parental wild-type Q/?, allowed an estimate of the mutation rate for a specific purine transition: about 10-4 per round of copying, a value about 105-fold larger than that estimated for the mutation rate of DNA genomes. Furthermore, genomic RNA from many phage clones was analyzed by T1-oligonucleotide fingerprinting (rapid nucleotide sequencing techniques were not yet available). The results indicated that, assuming a random distribution of mutations among the genomes analyzed, each infectious RNA differed in 1-2 positions from the average or consensus sequence in the population. Interestingly, the Tl-oligonucleotide map of the RNA from the uncloned phage population remained unchanged during 50 serial infections, a fact that must be kept in mind before considering stability of a consensus sequence as evidence against quasispecies. A highly dynamic mutant spectrum can nevertheless produce the same average sequence over many generations of viral replication. Weissmann and colleagues concluded that: 'The genome of Qf} phage cannot be described as a defined unique structure, but rather as a weighted average of a large number of different individual sequences.'
Subsequent studies with many human, animal and plant RNA viruses, or viruses which include an RNA
Mutant spec Ira
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