Biology 350: Microbial Diversity Strange Invaders: Viruses, viroids, and prions. Lecture #27 7 November 2007-1-
Notice handouts and announcements for today: Outline and study questions A 1999 paper discussing what the Archaeota have taught biology. Two papers describing a possible role for archaeans in human disease: one general and one specific to peridontal disease in humans. I enjoyed the e-mail questions---i hope you found my answers helpful. -2-
Another gentle and well intentioned reminder: You should be talking with me about issues involving your term paper. If possible, your paper should give autobiographical features of the prokaryote: genome, biochemistry, genetics, etc. Links on the web. Don t forget the one page version! Beware of last minute work! -3-
Archaeans and human disease: Plenty of commensal archaeota in association with eukaryotes. Methanogenic archaeans use hydrogen and carbon dioxide to make methane (and thus make energy). Hydrogen rich areas would be a good environment. If archaeans can be symbiotic, why not pathogenic? Polymicrobial disease concept. Relate to Koch s Postulates? -4-
Today s lecture agenda A VERY brief introduction to the world of the smallest microbial entities ---viruses. Some schizophrenia in this chapter, since the subject is so broad, and there exists emphases on bacterial and animal/plant viruses. Viruses ALWAYS reflect the nature of their host cells. -5-
First what is a virus? Consider a virus as a packet of information that forces a cell into making more virus. -6-
Nomenclature about viruses Some form of nucleic acid makes up the instructions (DNA or RNA, single or double stranded). Virus needs some kind of entry point to enter into host cells (receptors). Phage lambda (maltose receptor) versus HIV (CCR5 antigen on CD4 + T lymphocytes and some macrophages) examples. Turns host cell into a factory for making more virus. Virus particle = virion. Protein shell a capsid. -7-
Viruses: diversity of size and form Figure 17.1 Figure 17.2-8-
Viruses use host machinery to reproduce Figure 17.3 Enveloped virus of eukaryotic cells (note budding from membrane) Appears to use normal machinery of the cell. Note antigenic spikes that are antigenic to host. -9-
Viruses: some familiar viral faces -10-
Viral reproduction: steps in the process Attachment of virus to host cell (receptor-receptor recognition). Entry of virus into host cell. Activation/expression of viral nucleic acid. Shutting down normal host functions. Virus factory: making viral proteins, viral nucleic acids, and construction of new virions. Release of virions from infected cell. -11-
Viruses: challenges with nucleic acid Single stranded versus double stranded nucleic acid challenge. Positive versus negative strand definition. DNA versus RNA! -12-
Viruses: more challenges over nucleic acids Figure 17.6-13-
Challenges to an RNA genome. Positive strand RNA can be used as a mrna directly Negative strand RNA needs to be replicated again---requires a RNA directed RNA polymerase (virus carries enzyme!). Retroviruses like HIV need reverse transcriptase to convert RNA to DNA, and carry that enzyme, too! RNA genomes are error prone. -14-
Viral interaction with the host genome: the phage and lysogeny/lytic example. Figure 17.8 Two lifestyles available. Prophage/lysogen/provirus concept. Similar idea in some animal viruses (herpes simplex and fever blister example). Think about the viral advantage. -15-
Viral interaction with the host genome: the phage lambda/lysogeny connection. Figure 17.9 Notice attachment sites in phage and chromosome. Is lysogeny a good strategy? When? What signals control the process? -16-
Influenza: small, protean, and mighty! 1918-1919: Spanish Flu (actually from China) killed 50-100 million people worldwide. 2-20% fatal (normally 0.1% fatality rate). Caused by Type A influenza H1N1 Other episodes of influenza over the years, including the fear of bird flu. H = hemagglutinin: involved with virus binding to host cells. N = neuraminidase: involved in release of progeny virus from infected cells. Segmented negative RNA genome (8 segments). Note nomenclature of influenza: type, geographic origin, strain number, year of isolation, subtype. Subtype deals with variation in H and N antigens on the virion. -17-
The stranger behavior of influenza Influenza and a segmented RNA genome. RNA genome = sloppy polymerase = mutations (versus immune response). Segments = possible recombination among segments. Influenza epidemics: slight changes in H and N antigens on virion surface. Influenza pandemics: new virus created by coinfection and swapping of RNA genome segments ( bird flu ). -18-
Philosophical questions about viruses. Are viruses alive? How did viruses evolve? What are the impacts viruses have on evolution? -18-
Something stranger than viruses: viroids Figure 17.10 Tiny bit of single stranded RNA (forms three dimensional structure). Causes many plant diseases (coconut and chrysanthemum crop problems). What do viroids DO? Mysterious RNA codes for NOTHING. Interfere with splicing in host cells? Possible escaped intron that is fully selfish and self-replicating? Uses host RNA-directed RNA polymerase. Some viroids have ribozyme activity. -19-
Something stranger than viroids: prions Transmissible spongiform encephalopathies (TSE). PrP C versus PrP SC Alternative folding products of the same protein. PrP SC form creates long polymers and neurological degeneration. Can survive autoclaving. A recruitment effect converting PrP C into PrP SC. Transmissible rarely to humans. May be more common than we know (Chronic Elk Wasting Syndrome). -20-
Summing up today s lecture Viruses are rogue instructions that take over a host cell and turn it into a factory for making more virus. Great diversity in form and function. Great diversity in genome type and adaptations to that instruction set. Ability to work with as well as destroy host cell. Program is to make more virus---one way or another. Program is genetically highly regulated. Influenza an excellent example of a complex relationship with host cells and other viruses. Viroids are like escaped introns that cause some plants diseases. Prions are recruitment proteins capable of making more of themselves and cause disease. -21-
Topic(s) for next lecture Microbial Ecology, Part I Please read over Chapter 18 before our next class meeting! -22-