1. Life Cycle of M13 & Mu
Viruses
SHASHANK PATIL
171233, M.SC. BIOTECHNOLOGY-I

2. Introduction
 Filamentous structure, Single stranded positively
charged DNA with Protein capsid.
 Found useful in genetic engineering experiments.
 Gronenborn and Messing experiment- Assaying of β-
Galactosidase.
 Proves one of the best vector for single stranded DNA
synthesis.

3. Structure of M13 Virus
 Helical symmetry-Unusual character. ssDna acts as
template strand.
 Size of the genome is small – 6.4 kb.
 Phage length – 895 nm, thickness – 6 nm, external core
– 2.5 nm.
 Entire genome packed with GP8 proteins.
 Attachment end – five GP3 & GP6 proteins.
 Free end – five GP7 & GP9 proteins.
 IG region – b/n II & IV region – sequences for viral
packaging & +/- strand synthesis.

4. Electron micrograph of the filamentous M13 phage
Structure of M13 virus

5. Structure of M13 genome

6. Replication Cycle
 4 stages of replication – Attachment, Penetration,
Replication & Production of new viruses.
 Attachment – Binding of the phage to the bacterial
surface. Usually F – Plasmid `Male’ bearing sex pili.
 Penetration – Introduction of viral genome. GP6 & GP3
proteins bind to pilus and GP8 to plasma membrane
by conformational changes.
 Other proteins help in the replication and post –
replication processes.

7.  Replication – 2 types, positive & negative strands.
 B & C – ori sites for negative strand
 D & E – ori sites for positive strand
 Negative strand synthesis – ssDNA with coated ssB
proteins – super coiled. RNAP binds to the C region. IG
region acts as a recognition site by hairpin loop
formation. dsDNA/RF DNA is synthesized.
 Positive strand synthesis – RF forms undergo rolling
circle mechanism to produce the positive strand. GP2
binds to D region. As the ssDNA is produced, viral
protein binds and stops this mechanism.

8.  mRNA thus produced from RF form further produces
viral proteins.
 Increased GP5 produces nick in the DNA – in the IG
region. ssDNA produced.
 GP1 creates ion channels on the surface of the plasma
membrane and GP4 help in the assemblage of DNA.
 GP5 binds to DNA to avoid the damages from host
nucleases. GP8 covers the whole strand to protect it.
 GP4 also helps to bind the DNA to all the proteins.
 GP7 & GP9 – first addition. GP3 & GP6 – last addition.
 Thus without harming the host cell mechanism, the
newly formed DNA passes out through the plasma
membrane.

10. Life cycle of Mu virus

11. Introduction
 Known as Mu phage or Mu bacteriophage. Since it
affects Enterobacteria, also called as Enterobacteria
phage.
 Named after its ability to cause mutations in life cycle.
 Replicates through transposition of gene sequences.
 Broad host range – research areas.
 Temperate phage – displays both lytic and lysogenic
phases.

12. Structure of Mu virus
 Icosahedral head, tail and 6 tail fibers.
 Helical symmetry and devoid of envelope.
 Head – 54 nm, pro-late in shape.
 Capsids are hexagonal in shape. 152 capsomers.
 Long tail, rigid and thick & consists an axial canal, base
plate and fibers.
 Sheath composed of stacked rings, upon contraction
which becomes shorter and thicker.

13. Scanning Phage Mu Electron Micrograph of Mu

14. Genome of Mu virus
 Non-segmented, single, linear dsDNA.
 37 kb long but actual length is 35.6 kb. Rest genome –
host DNA.
 Completely sequenced genome – 37,611bp long.
 DNA integration at one point of host chromosome
helps in lysogeny of the host cell.

15. Life Cycle of Mu virus
 Infection through attachment – Virus attaches on the
surface of the host bacteria.
 Penetration – genome is inserted to the DNA of the
host cell through cut and paste mechanism.
 Protected from host restriction by acetylation of
Adenine residues.
 After the infection, phage Mu enters either lytic or
lysogenic cycle – dependent on repressor protein Rep.
 Linear genome is replicated as a part of the host DNA.

16.  Host DNA acts as a target site for the binding site after
the duplication of 5 base pairs.
 Staggered cuts are made, which permits the
integration of the viral DNA.
 Single strands are converted to double strands – by
Mu integration. Requires transposase – gene A
product.
 Mu repressor – Gene C product - not activated – lytic
pathway. Can’t enter if lysogeny is introduced.
 Mu DNA is replicated by repeated transposition of
multiple sites on host genome.
 Viral protein synthesis – Protein C expression – head
and tail formation.

17.  Cell is lysed – mature phage particles produced.
 Accumulation of repressor protein – transcription of
Mu integrated DNA prevented – Lysogenic state.

18. References
 Brock Biology of Microorganisms – Madigan. Martniko.
Stahl. Clark – 13th Edition.
 Website – NCBI
 Biologydiscussion.com
 Viralzone.expassy.org
 Mol.biol4masters.masters.grkraj.org
 Utminers.utep.edu