The document discusses the structure of two viruses: Tobacco Mosaic Virus (TMV) and bacteriophage T4. TMV has a rod-like structure composed of 2130 coat protein molecules assembled helically around a single-stranded RNA genome. It is approximately 300nm long and 18nm wide. T4 is a tadpole-shaped bacteriophage that infects bacteria. It has a hexagonal protein head connected to a helical tail by a collar. The tail ends in a base plate with spikes and fibers that attach to bacterial cells. Both viruses have genomes protected within their protein capsids during replication.

1. Microbial Taxonomy and Diversity –
Structure of Virus
Dr. S. Sivasankara Narayani
Assistant Professor
Department of Microbiology
Ayya Nadar Janaki Ammal College
Sivakasi
01-10-2020
Dr.SS

2. UNIT–III
CLASSIFICATION
OFVIRUS
 Viruses: Physical, chemical and biological properties.
Baltimore classification. Types of viruses based on
structure and Host. Cultivation of viruses: Cell line -
embryonated eggs - cytopathic effect. Structure of
viruses: TMV- T4bacteriophage. Replication of virus:
Lytic and lysogenic cycle. Phage typing.
01-10-2020
Dr.SS

3. TMV
 Tobacco mosaic virus has a rod-like appearance.
 Its capsid is made from 2130 molecules of coat protein
 one molecule of genomic single strand RNA, 6400 bases long.
 The coat protein self-assembles into the rod-like helical structure (16.3
proteins per helix turn) around the RNA, which forms a hairpin loop
structure.
 The protein monomer consists of 158 amino acids which are assembled
into four main alpha-helices, which are joined by a prominent loop
proximal to the axis of the virion.
 Virions are ~300 nm in length and ~18 nm in diameter.
 Negatively stained electron microphotographs show a distinct inner
channel of radius ~2 nm.
 The RNA is located at a radius of ~4 nm and is protected from the
action of cellular enzymes by the coat protein.
 X-ray fiber diffraction structure of the intact virus was studied based
on an electron density map at 3.6 Å resolution.
 Inside the capsid helix, near the core, is the coiled RNA molecule,
which is made up of 6,395 ±10 nucleotides

4. Structure

5. Genome
 The TMV genome consists of a 6.3–6.5 kb single-
stranded (ss) RNA.
 The 3’-terminus has a tRNA-like structure, and the 5’
terminus has a methylated nucleotide cap.
(m7G5’pppG).
 The genome encodes 4 open reading frames (ORFs),
two of which produce a single protein due
to ribosomal readthrough of a leaky UAG stop codon.
 The 4 genes encode a replicase (with methyltransferase
[MT] and RNA helicase [Hel] domains), an RNA-
dependent RNA polymerase, a so-called movement
protein (MP) and a capsid protein (CP).[18]

6. T4
Bacteriophage
 Viruses infecting bacteria are called Bacteriophages . It literally
means ‘eaters of bacteria’ (Gr: Phagein = to eat). Phages are
abundant in soil, sewage water, fruits, vegetables , milk
 The T4 phage is tadpole shaped and consists of head, collar, tail,
base plate and fibres.
 The head is hexagonal which consists of about 2000 identical
protein subunits.
 The long helical tail consists of an inner tubular core which is
connected to the head by a collar.
 There is a base plate attached to the end of tail.
 The base plate contains six spikes and tail fibres.
 These fibres are used to attach the phage on the cell wall of
bacterial host during replication.
 A dsDNA molecule of about 50 µm is tightly packed inside the
head. The DNA is about 1000 times longer than the phage itself

7. Structure

8. Genome
 Phage T4 has provided countless contributions to the
paradigms of genetics and biochemistry. Its complete
genome sequence of 168,903 bp encodes about 300 gene
products.

9. Clarifications
sivasan91@gmail.com
01-10-2020
Dr.SS

10. Questionsto
think
 Structure of TMV
 Structure of T4
01-10-2020
Dr.SS

11. 01-10-2020
Dr.SS