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

## TL;DR

This study shows how viruses can replicate using proteins made in very low amounts due to gene-expression errors, which might help them survive in new hosts.

## Contribution

The paper experimentally demonstrates virus replication enabled by exceedingly low levels of a host protein due to gene-expression errors.

## Key findings

- Phage T7 can replicate even when only about 10 thioredoxin molecules are present per host cell.
- Low thioredoxin levels are possible due to the polymerase-thioredoxin complex's high kinetic stability and long residence time.
- Gene-expression errors may allow viruses to access diverse protein variants, aiding survival in new hosts.

## Abstract

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.