Plant Viruses

Volume I: Structure and Replication

C.L. Mandahar

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780367412241
• eBook ISBN: 9781351092692
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This book deals with the structure and replication of plant viruses, viroids, satellites of plant viruses, and spiroplasmas and neatly sums up the state of our knowledge about these aspects of these pathogens.Published information about single-stranded positive-sense RNA plant viruses is far greater than for any other group of plant pathogens.The book caters to the needs of students as well as researchers and is illustrated with micrographs, figures of postulated models, and genetic maps

Chapter 1
Monopartite Elongated Viruses

C. L. Mandahar

Table of Contents

I. Introduction

II. Tobamovirus Group

A. Tobacco Mosaic Virus

1. Capsid

a. Protein Aggregates

b. Fine Structure of Polypeptide Subunit in the Virion

c. Fine Structure of Polypeptide Subunit in the Disk

2. Genome

3. Virion Stablizing Bonds

B. Other Tobamoviruses

III. Potexvirus Group

IV. Carlavirus Group

V. Potyvirus Group

A. Virus Particles

B. Capsid

C. Genome

1. Genetic Map

2. Nucleotide Sequence

VI. Closterovirus Group

References

I. Introduction

The monopartite elongated viruses containing single-stranded RNA genome of positive polarity have been placed in five groups: tobamoviruses, potexviruses, carlaviruses, potyviruses, and closteroviruses (Table 1). The struction of these viruses is discussed in this chapter.

II. Tobamovirus Group

Only the structure of tobacco mosaic virus is discussed in detail.

A. Tobacco Mosaic Virus

Tobacco mosaic virus (TMV) was the first virus to be crystallized and also the first virus to be studied using an electron microscope. Since then much information on its structure has been published. The earlier work has been reviewed several times.^1-7 X-ray diffraction of oriented gels of TMV virions to a resolution approaching 4 Å, and of crystals of TMV protein disks to a resolution of 2.8 Å, have mainly contributed to our new knowledge about fine TMV structure.^8,9 An atomic model of TMV structure has been built on the basis of these studies. A model of intact TMV virions based on X-ray fiber diffraction studies at 3,6 Å resolution has also been proposed.¹⁰ The TMV structure at the atomic level has also been reviewed.^10-14

The TMV particles consist of coaxially intertwined protein and RNA to produce helical, grooved, cylindrical, rigid rods of remarkably uniform dimensions of 300 nm length and 18 nm diameter. A hollow channel of 4 nm diameter runs throughout the entire length of the rod and is enclosed by 14 nm thick walls of the cylinder. Virus protein comprises 95% by weight while RNA constitutes the remaining 5%.

Virus particle is composed of a right-handed helical organization of protein subunits disposed of along the long axis of the particle. The pitch of the helix is 2.3 nm while each axial repeat of the helix is 6.9 nm. There are 16⅓ protein subunits per turn of the helix so that 49 protein subunits are present for every 3 turns of 6.9 nm. The total number of turns in a virion is about 30. The RNA is located at a distance of 4 nm from the particle axis and is deeply embedded between successive turns of the helix.

1. Capsid

The capsid of each TMV particle is composed of about 2130 identical protein subunits. Each protein subunit is tapered on the outside and possesses two grooves, one on the outer side and second on the inner side. The two grooves Of all the protein subunits together give rise to two furrows which, following the helical arrangement of subunits, also have helical symmetry. Each protein subunit has a molecular weight of 17.5 × 10³ and is composed of 158 amino acid residues. The primary sequence of amino acid residues of TMV protein subunit is known.^15,16

The TMV protein subunit lacks methionine and histidine. The N-terminal amino acid residue is normally acetylated serine. However, N-terminal amino acid residue in TMV strain U2 is proline and is not acetylated. The C-terminal amino acid residue is threonine. The only cysteine residue is situated at position 27 which is deeply buried in the virion structure and is located at a radial distance of 5.6 nm from the particle axis.

Total ionizing groups in the TMV protein subunit chain are 16 acidic groups, 13 basic groups, 4 phenolic groups, and 1 sulfhydryl group. Out of these, 8 acidic and 7 basic residues are involved in interactions contributing to virion structure and are not therefore surface oriented but are hidden and unreplaceable.

Of the 13 basic amino acid residues 2 are lysines; they are located at positions 53 and

Table 1
GROUPS OF ELONGATED MONOPARTITE VIRUSES CONTAINING SINGLE-STRANDED RNA GENOME OF POSITIVE-SENSE

		Molecular weight
Virus group	Particle dimension (nm)	RNA (× 10^6)	Capsid protein subunit (× 10^3)	Other characteristics
Tobamovirus	300 × 18	2.0	17–18	Rigid helical rods with 2.3 nm pitch and clearly visible axial canal.
Potexvirus	470–580 × 13	2.0–2.1	18–27	Slightly flexuous helical rods with 3.4 nm pitch, clear cross bands, protein subunits often occurring in longitudinal flies, and occasionally visible axial canal.
Carlavirus	610–700 × 13	2.3–3.0	30	Slightly flexuous helical rods with 3.4 nm pitch, often curved to one side, four long-itudinal furrows separating files of subunits, cross-banding (primary helix) and axial canal often not visible.
Polyvirus	720–850 × 11	3.0–3.5	32–36	Flexuous helical rods with 3.4 nm pitch, fine cross-banding usually visible, axial canal often not seen.
Closterovirus	700–2000 × 10	2.3–6.5	23	Very flexuous open helical rods with 3.7 nm pitch, primary helix clearly visible, axial canal not seen.

68. Lysine 53 forms ionic interactions with a negative group which could be one of t...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Chapter 1 Monopartite Elongated Viruses
• Chapter 2 Monopartite Spherical Viruses
• Chapter 3 Multicomponent Viruses
• Chapter 4 Multiplication of Plus-Sense RNA Viruses
• Chapter 5 Reconstitution of Plant Viruses
• Chapter 6 Plant Reoviruses
• Chapter 7 Plant DNA Viruses
• Chapter 8 Plant Viroids: A Biochemical Novelty
• Chapter 9 Satellites of Plant Viruses
• Chapter 10 Spiroplasmas and Mycoplasmalike Organisms
• Index