The genome of Qβ is approximately 4,217
nucleotides, depending on the source which sequenced the virus. Qβ has been isolated all over the world, multiple times, with various subspecies that code for nearly identical proteins but can have very different nucleotide sequences. The genome has three
open reading frames that encode four
proteins: the maturation/
lysis protein A2; the
coat protein; a readthrough of a leaky
stop codon in the coat protein, called A1; and the β-subunit of an
RNA-dependent RNA-polymerase (RdRp) termed the replicase. The genome is highly structured, regulating
gene expression and protecting itself from host
RNases. The structure of the Qbeta
RNA replicase has been solved (). The two EF proteins serve as a
chaperone for both the replicase and the RNA product. In fact, pure Qbeta polymerase is not soluble enough to be produced in large quantities, and a fusion protein constructed from the replicase and the two EF subunits is usually used instead. The fusion can function independently of ribosomal protein S1.
Maturation/lysis protein A2 All positive-strand RNA phages encode a maturation protein, whose function is to bind the host
pilus and the viral RNA. The maturation protein is named thus, as
amber mutants in the maturation protein are unable to infect their host, or are 'immature.' For the related +ssRNA
bacteriophage MS2, the maturation protein was shown to be taken up by the host along with the viral RNA and the maturation protein was subsequently cleaved. In
bacteriophage MS2, the maturation protein is called the A protein, as it belongs to the first open reading frame in the viral RNA. In Qβ the A protein was initially thought to be A1, as it is more abundant within the virion and is also required for infection. However, once the sequence of Qβ was determined, A1 was revealed to be a readthrough of the leaky stop codon. A2 is the maturation protein for Qβ and has an additional role of being the lysis protein. The mechanism of lysis is similar to that of
penicillin; A2 inhibits the formation of
peptidoglycan by binding to MurA, which catalyzes the first enzymatically committed step in cell wall biosynthesis. ==Experiments==