Micro Lecture #13 notes p.330-359 Viruses

early 1900s; Twort-d'Herelle phenomenon "bacteria eaters"(bacteriophage) Filterable

Host range = for many years most important for classification (utilitarian again?). Now: Nature of genetic material.

Protomers, capsomers, icosohedral capsids, enveloped vs. naked

Adsorption, penetration, uncoating, release

Plating efficiency (%viability for viruses)

Helical, polyhedral, complex viruses

Lytic and lysogenic (temperate, prophage-forming) cycles; specialized and general transduction

T4, lambda, Q beta, mu = models

HIV, reverse transcriptase, duplicate RNA genome, evolution, and retroviral hypermutability

Influenza viruses, Chinese ducks, pigs, hemagglutinin, neuraminidase, antigenic drift/shift

Protooncogenes, oncogenes and viral genetic theft

Viroid = ssRNA alone. Prion = protein template for misfolding (Ice-9) Mad cow disease, Crutzfelt-Jacob syndrome, greater susceptibility of some individuals.

http://web.uct.ac.za/depts/mmi/stannard/virarch.html

The complex arrangements of macromolecules in the virus shell are minute marvels of molecular architecture. Specific requirements of each type of virus have resulted in a fascinating apparent diversity of organization and geometrical design. Nevertheless, there are certain common features and general principles of architecture that apply to all viruses.

In 1956, Crick and Watson proposed on theoretical considerations and on the basis of rather flimsy experimental evidence then available, principles of virus structure that have been amply confirmed and universally accepted. They first pointed out that the nucleic acid in small virions was probably insufficient to code for more than a few sorts of protein molecules of limited size. The only reasonable way to build a protein shell, therefore, was to use the same type of molecule over and over again, hence their theory of identical subunits The second part of their proposal concerned the way in which the subunits must be packed in the protein shell or capsid. On general grounds it was expected that subunits would be packed so as to provide each with an identical environment. This is possible only if they are packed symmetrically. Crick and Watson pointed out that the only way to provide each subunit with an identical environment was by packing them to fit some form of CUBIC SYMMETRY. A body with cubic symmetry possesses a number of axes about which it may be rotated to give a number of identical appearances. These predictions were soon confirmed and it became evident that the occurrence of icosahedral features in quite unrelated viruses was not a matter of chance selection but that icosahedral symmetry is preferred in virus structure.

An ICOSAHEDRON

is composed of 20 facets, each an equilateral triangle, and 12 vertices, and because of the axes of rotational symmetry is said to have
5:3:2 symmetry

Axes of Symmetry

There are, in fact, six 5-fold axes of symmetry passing through the vertices, ten 3-fold axes extending through each face and fifteen 2-fold axes passing through the edges of an icosahedron

Icosahedral symmetry requires definite numbers of structure units to complete a shell. In their discussions, Crick and Watson (1956), thinking in terms of asymmetrical protein subunits packed in such a way that each has an identical environment, pointed out that a virus with 5:3:2 symmetry required a multiple of 60 subunits to cover the surface completely. Each unit would be related identically and asymmetrically with its neighbours, and none of the units would coincide with an axis of symmetry.

For a curious discussion of whether HIV causes AIDS:

http://www.virusmyth.net/aids/data/cjinterviewep.htm