Chemical composion of virus

The simplest virions consist of two basic components: nucleic acid (single- or double-stranded RNA or DNA) and a protein coat, the capsid, which functions as a shell to protect the viral genome from nucleases and which during infection attaches the virion to specific receptors exposed on the prospective host cell.

1. WEL COME Mr. Sandesh V. Pawar Dept. of Plant Pathology Dr. PDKV., Akola

2. Chemical composition and Structure of plant viruses

3. The main chemical composition of plant viruses are Protein Nucleic acid (either DNA or RNA never both) A) Protein Major structural component of plant viruses. Serving as a protective coat for the delicate nucleic acid genome . It provides the basis for serological differentiation of plant viruses. Proteins are composed of about 150-600 or more residues of some 20 amino acid .

4. Most plant viruses are built of one kind of protein sub units. In protein contain amino acid , and they carry an amino group (-NH2) and the carboxyl group (-COOH) of atom carrying the chemical residues as a side chain. The amino acid condensed to polymerize into a linear polypeptide chain. The specific sequence of amino acid , linked by peptide bonds, is the first level of protein organization and called Primary structure . The polymerized peptide structures are usually spirally or helically bound to form the Secondary structure.

5. Further bending or folding of polypeptide chain over itself is called Tertiary structure . The linear polypeptide has two ends that is C-terminal end and N-terminal end . The primary structure of protein molecules results from strong covalent peptide bonds (-CO-NH-). The secondary and tertiary structures are maintained by a few strong covalent disulphide (-S-S-) bonds but mostly by a large number of weak hydrogen bonds . There are few plant viruses such as potato yellow dwarf and rhabdoviruses which contain complex protein configuration containing more than one type polypeptides and forms quaternary structure.

6. Amino acids Alanine (Ala) Arginine (Arg) Aspartic acid (Asp) Asparagine (Asn) Cysteine (Cys) Glutamine (Gln) Glutamic acid (Glu) Glycine (Gly) Histidine (His) Isoleucine (Ile) Leucine (Leu) Lysine (Lys) Methionine (Met) Phenylalanine (Phe) Proline (Pro) Serine (Ser) Tryptophan (Trp) Tyrosine (Tyr) Threonine (Thr) Valine (Val)

7. The capsid proteins fundamentally do not differ from other types of proteins in any way except that they may be deficient in some commonly occurring amino acid such as histidine and methionine in TMV and tryptophane in closteroviruses . The organizational and constitutional configuration of protein molecules of capsid confers it resistance to enzyme like protease and high temperature . The protective role of viral protein has two very important biological functions, determining serological affinity and vector transmission .

8. Nucleic acid The plant viruses contain either DNA or RNA as a genetic material. Most of the plant viruses contain RNA except a few such as caulimoviruses , geminiviruses , banana bunchy top and badnaviruses which contain either single or double stranded DNA . The nucleic acid may be presents as a single continuous strand in a particle or may be distributed in two or more pieces in different particles are called multipartite virus or divided genome .

9. The RNA and DNA may again be either single stranded or double stranded . Most of the plant viruses are ssRNA with a few exceptions such as clover wound tumor and rice dwarf virus which have dsDNA . The structural configurations of nucleic acid are similar to protein in the form of primary, secondary and tertiary structures. The sequence of nucleotides into a long poly-nucleotide chain constitutes the primary structure . The ssRNA in a free state folds upon itself into loop or helix due to transmolecular hydrogen bonding between A, U and G, C base pair these structure called secondary structure.

10. Viral Structure Morphologically the plant viruses fall into two broad categories; Anisometric (Rigid or flexous rods and bullet shaped) Isometric ( Polyhedral shaped)

11. Flexuous thread like virus Rigid rod shaped Side arrangement of protein subunit Short bacillus like virus Isometric polyhedral virus

12. Rod shaped particle All rods are helical in structure. The rod shaped viruses may be rigid rods as are TMV particles, or flexuous like the potyvirus . Rigid rod shaped TMV is most widely studied virus in this group. The entire TMV particle is very stable due to interaction between protein sub unit and RNA. The particle measures about 300 nm x 15 nm . The molecular weight of particle is 39.4 X 10 6 daltons

13. It consist of about 2100 helically arranged identical protein sub units along with axial canal. Each with molecular weight of 17500 Daltons and consisting of 158 amino acid residues. The pitch of the helix is 2.3 nm and the particle structure repeats every 6.0 nm of its length each three turns of the helix. The nucleic acid strands has a molecular weight of 2 x 10 6 Daltons containing some 6400 nucleotides which follows the pitch of helix and is embedded between the protein sub units 4 nm from the particle axis.

14. Flexuous rods The particles are not straight but show more or less marked bending . The particle of potexviruses have flexuous particle 480 to 580 nm long and 11 to 13 nm with obvious cross bandings. The particle have basic helix pitch of 3.3 nm and 3.6 nm depending on the dryness and wetness of the particle respectively. The particle of carlaviruses have straight and rigid structure but having a slight bend at one end. They measures 650 nm in length and 12 nm in width with bandings.

15. Isometric particle The isometric or icosahehedral virus particle are complex in their structure and architecture. An icosahedron is a polyhedron with 20 equilateral triangular faces. It shows three fold symmetry when rotated around the axis through the center of each triangular face. There are 12 vertices when the five triangle concern meet and thus 60 asymmetric units are required to construct an icosahedron . The surface of an icosahedron are divided into large number of smaller identical triangle called triangulation number.

16. A simple icosahedron having three triangulation number and nine triangle faces may have 180 sub units These sub units are clustering in different ways, having three clustering pattern 1) Trimer capsomere :- 3 sub units cluster. 2) Pentamer capsomere :- Sub units cluster at a vertice where faces of five triangles meet. 3) Hexamer capsomere :- Sub units cluster at a vertice where faces of six triangles meet.

17. Virus Particles with Membrane Spherical particle Tomato spotted wilt virion has rounded irregular shape, measuring about 70 nm diameter in situ. The particle resemble the leuko viruses of animal. The membrane binding the particle is about nm thick which is composed of protein sub units. It has been suggested that the tomato spotted wilt virus particle is pleomorphic myxovirus . Another virus in this group is carrot mottle having more or less rounded particles which measures about 50 nm in diameter.

18. Bacilliform or Bullet shaped particles This group of plant viruses are of complex structure. They resemble some animal viruses such as vesicular stomatitis and cocal virus. The group includes plant viruses such as maize mosaic virus, lettuce necrotic yellows and sow thistle yellow vein. The particle of this viruses are very fragile. Most are 300 to 400 nm long and 50 to 80 nm wide. The membrane around the helical core is lipidwhich might be acquired by core while passing through the host membrane.

19. Geminate particles Gemini virus are unique plant pathogenic group of viruses having geminate or twin particle( dimer ) constituted by two icosahedral monomers. The two monomers are joined at the discontinuities created by missing capsomers to give the characteristics flattened appearance at the interface. The dimer particle shows groove at the centre of its interface.