Enterobacteria phage Qbeta

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Enterobacteria phage Qbeta
Bacteriophage Qβ attached to sex pilus of E. coli.jpg
Virus classification
Group IV ((+)ssRNA)
Bacteriophage Qβ
  • Enterobacteria phage M11
  • Enterobacteria phage MX1
  • Enterobacteria phage ST
  • Enterobacteria phage VK

Bacteriophage Qβ has a single-stranded RNA linear genome packaged in an icosahedral capsid with a diameter of 25 nm. Its host is Escherichia coli. Qβ enters its host cell after binding to the side of the F pilus.[1]


The genome of Qβ is 4215 nucleotides long. The genome has three open reading frames and encodes four proteins: A1, A2, CP and qβ replicase. The genome is highly structured, which regulates gene expression and protects the genome from host RNases.[1] A1, the minor caspid, is produced by a stop codon readthrough of CP.[2]

Protein A2[edit]

A2 is called the maturation protein due to its lysis activity. One copy of A2 is present per virion. The mechanism of lysis is similar to that of penicillin; A2 inhibits the formation of peptidoglycan by inhibiting the enzyme UDP-N-acetylglucosamine 1-carboxyvinyltransferase (MurA), which catalyzes the first committed step of peptidoglycan synthesis. A2 also functions in host cell recognition and cell entry. When A2 binds to the sex pilus of the bacterium, A2 cleaves and forms a pore into the host.

The structure of A2 bound to MurA has been solved by fitting X-ray crytallography data to EM observations (PDB: 5vm7​). In the final caspid it presents as a spike (PDB: 5VLZ​).[3]

RNA Polymerase[edit]

The RNA polymerase that replicates both the plus and minus RNA strands is a complex of four proteins: the cayalytic beta subunit (replicase, P14647) is encoded by the phage, while the other three subunits are encoded by the bacterial genome: alpha subunit (ribosomal protein S1), gamma subunit (EF-Tu), and delta subunit (EF-Ts).[4]

The structure of Qbeta RNA replicase has been solved (PDB: 3AGP, 3AGQ​). The two EF proteins serve as a chaperone for both the replicase and the RNA product.[5] 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.[6]


RNA from Bacteriophage Qβ was used by Sol Spiegelman in experiments that favored faster replication, and thus shorter strands of RNA. He ended up with Spiegelman's Monster, an minimal RNA chain of only 218 nucleotides that can be replicated by Qβ replicase.


  1. ^ a b Kashiwagi A, Yomo T (August 2011). "Ongoing phenotypic and genomic changes in experimental coevolution of RNA bacteriophage Qβ and Escherichia coli". PLoS Genetics. 7 (8): e1002188. doi:10.1371/journal.pgen.1002188. PMC 3150450. PMID 21829387.
  2. ^ "Allolevivirus". ViralZone. Retrieved 13 May 2019.
  3. ^ Cui, Z; Gorzelnik, KV; Chang, JY; Langlais, C; Jakana, J; Young, R; Zhang, J (31 October 2017). "Structures of Qβ virions, virus-like particles, and the Qβ-MurA complex reveal internal coat proteins and the mechanism of host lysis". Proceedings of the National Academy of Sciences of the United States of America. 114 (44): 11697–11702. doi:10.1073/pnas.1707102114. PMID 29078304.
  4. ^ van Duin, J.; Tsareva, N. (2006). "Single-stranded RNA phages. Chapter 15". In Calendar, R. L. (ed.). The Bacteriophages (Second ed.). Oxford University Press. pp. 175–196. ISBN 0195148509.
  5. ^ Takeshita, D; Tomita, K (7 September 2010). "Assembly of Q{beta} viral RNA polymerase with host translational elongation factors EF-Tu and -Ts". Proceedings of the National Academy of Sciences of the United States of America. 107 (36): 15733–8. doi:10.1073/pnas.1006559107. PMID 20798060.
  6. ^ Kita, H; Cho, J; Matsuura, T; Nakaishi, T; Taniguchi, I; Ichikawa, T; Shima, Y; Urabe, I; Yomo, T (May 2006). "Functional Qbeta replicase genetically fusing essential subunits EF-Ts and EF-Tu with beta-subunit". Journal of bioscience and bioengineering. 101 (5): 421–6. doi:10.1263/jbb.101.421. PMID 16781472.

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