Phenotypic modification and restriction can also be the consequence of molecular processes not directly involving the DNA but rather certain viral proteins. They were termed 'nonclassical' to distinguish them from the 'classical', purely DNA-based mechanisms.
In nonclassical M/R, which was first described for phage T3, the ability of a phage to adsorb to a given host cell is affected by the prior host strain. Representatives of the genetically unaltered virus varying in their adsorption-protein modification also differ serologically. The molecular mechanism has not been resolved yet.
In eukaryotic viruses so far all host cell adaptation is explained by the appearance and selection of viral mutants. It has, however, not been investigated whether phenotypic M/R processes of genetically unaltered viruses do also occur in the kingdom of eukaryotes, possibly explaining the occurrence of reversible changes in the host cell tropism of viruses.
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