Qb Virus:


These are bacteria eating viruses and they belong to RNA phages, which are classified into 3 serologic groups.



RNA Bacteriophages:


Viral group

Specific viruses



F2, fr, FH5, MS2, R17, M12,



Ff, If












Qb RNA phages are extremely small viruses, 25 nm thick, isometric, adsorb to specific bacterial sex pili and infect.   So they are male dependent virus.  The phage has 20 faces and 12 vertices. Each face is made up of six subunits and each vertices is made of five subunits; in all it has 180 protein subunits. Inside the capsid a single stranded genomic RNA is packed; it is a linear molecule but extensively folded into secondary structures.


Qbeta viruses related to another similar virus called MS2, whose genome is more or less same as that of Qbeta.


Q-beta  phage particles










Infection is through sex pili.  They bind to sex pili in large numbers. Through the hallow of the sex pili the RNA genome is transported leaving behind the empty capsid!  Once infected, they can easily replicate and produce about 5000 to 10000 particles per cell in about 60 minutes. Though the infection is through sex pili, the exact mechanism of the genome transfer is not known.  The genome is linear, but it has very high level of secondary structures (62 to 80% of the genome), with many hairpin like loops, even at 5’ and 3’ ends.  In fact the secondary structure provides protection from cellular exonuclease digestion.  The length of the RNA is 3600-4105 (?) ntds, and codes for four proteins.  The RNA has positive sense.  At 5’ end it has ppp5’G-GGU and at 3’ end it has CCA 3’OH, both ends have hairpin structures.


·         Coat protein = 13.7 KD, there are 180 subunits per phage.

·         A1 protein = 38 KD, extension of coat protein. Only one copy per virus, it is required for attachment of the phage to virus.  It is a read through product of the coat gene.


·         A 2 protein = Maturation protein, also has lytic function.  It is required for the maturation of the phage i.e. it is required for the assembly of the particles within the host cell and ultimately leads to the lysis of cells.


·         b1 = Replicase, 61-65 KD.  It resembles to that bacterial beta sub unit, with template site and an active site.  But it requires few more proteins from the host cell for its activity.


·         Coat proteins are produced in greater abundance than the others, the ratio is, 20 coat protein: 5 b protein: 1 A2


·         A2 gene = initiator codon is GUG, terminator codon is UAG.


·         Coat protein= AUG is the initiator, UGA or UAA are terminators, UGA act as read through codon for the synthesis of A1 protein.


·         Replicase (beta) = AUG is the initiator, terminator is UAG.



ppp5’G-GGU—62------1322-1345----1744----2332-2353-----4120-4220-3’ Qb



Coli phage MS2, Monopartite, linear, ssRNA(+) genome about 4 kb in size. The 5’ end is capped. Encodes 4 proteins. http://viralzone.expasy.org/





Alloleviviral RNA, q-beta-http://www.lookfordiagnosis.com



From 1-62, 5’ end leader sequence.

From 62 – 1322 ntd, A2, maturation protein (75 aa long).

From 1345 – 1744 ntd, coat protein (129 aa long).

From 1345 – 2332 ntd, A1 attachment protein (353 aa long).

From 2353 – 4120 ntd, Replicase protein.

From 4120 – 4220 ntd, 3’ end noncoding terminal sequence.

From 1322 to 1345, from 2332 to 1353 are noncoding spacers.



Qb - RNA replication enzyme:


The replication enzyme is called RNA dependent RNA polymerase or RNA Replicase.   The replicating enzyme is a complex made up of two virally coded b protein and few host proteins.








Mol.wt (subunits) in KD


Beta protein

60-61 (544 aa)

Viral coded, has enzyme properties

Alpha subunit

70  (host’s small

ribosome s10)

One of the host small ribosomal subunit

Gamma protein


Bacterial elongation factor EF-Tu



Bacterial EF-Ts


72 (hexamer)

Associated with ribosome






The genomic RNA, before initiation of replication, undergoes translation. Initiation of translation takes place at an AUG codon found in non-hairpin region.














(+) RNA-    5’ UGG--------------------------------------------->CCA 3’



(+) RNA-    5’ UGG--------------------------------------------->CCA 3’

(-)  RNA-    3’ ACC<----------------------------------------------GGU 5’


(+) RNA-    5’ UGG---------------------------------------------->CCA 3’



Life cycle




Replication of Q-beta RNA, first the +RNA is copied by its Qb-Replicase enzyme, then the –strand is copied to + strand; http://www.lookfordiagnosis.com


Infection, growth and lyses