Virus Propagation Linked to Exceedingly Rare Gene-Expression Errors: A Single-Molecule Microscopy Demonstration

Many viruses use programmed frameshifting and stop-codon misreading to synthesize functional proteins at high levels. The underlying mechanisms involve complex RNA sequence/structure motifs and likely reflect optimization driven by natural selection of inefficient, nonprogrammed processes. Then, it follows from basic evolutionary theory that low levels of proteins generated through gene expression errors could provide viruses with some survival advantage. Here, we devise an experimental demonstration of this possibility. Phage T7 recruits the host thioredoxin as an essential processivity factor for the viral DNA polymerase. We inserted early stop codons in the thioredoxin gene and appended to its end the sequence encoding for a photoconvertible fluorescent protein. Virus replication was not abolished. Single-molecule localization microscopy showed that the phage replicates even when there are only about 10 thioredoxin molecules per host cell on average, a number orders of magnitude below typical cellular protein levels. We show that this seemingly shocking result can be understood in molecular and evolutionary terms as a consequence of the polymerase-thioredoxin complex displaying high kinetic stability and a long residence time, as these are required to ensure high polymerase processivity. More generally, our demonstration that virus replication may be enabled by proteins at exceedingly low copy number suggests that viruses have access to the wide diversity of protein variants harboring phenotypic mutations as a result of gene expression errors. This mechanism could play a role, for instance, in cross-species transmission by enabling virus survival in the new host before adaptations appear at the genetic level.