1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viruses of Bacteria Bio 261 Microbiology Medgar Evers College Prof. Santos

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recall that bacteria are prokaryotes – With cells much smaller and more simply organized than those of eukaryotes! can someone give me some differences between eukaryotes and prokaryotes! Viruses – Are smaller and simpler still! 0.25  m Virus Animal cell Bacterium Animal cell nucleus

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Viruses Viruses – Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope. – A virus has a genome but can reproduce only within a host cell

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Genomes Viral genomes may consist of – Double- or single-stranded DNA – Double- or single-stranded RNA

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 18  250 mm 70–90 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses RNA DNA Capsomere Glycoprotein Capsomere of capsid Capsids and Envelopes A capsid – Is the protein shell that encloses the viral genome – Can have various structures

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Capsids are formed from identical protein subunits called capsomeres. Capsid + nucleic acid = nucleocapsid

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some viruses have envelopes – Which are membranous coverings derived from the membrane of the host cell 80–200 nm (diameter) 50 nm (c) Influenza viruses RNA Glycoprotein Membranous envelope Capsid

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Spike proteins present on the capsid allow for attachment to host. In the enveloped viruses, just inside the lipid envelope there is often matrix proteins, which are found only on enveloped viruses. The attachment spikes project from the envelope.

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A naked virus is one that does not have an envelope. Virtually all phages are naked.

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bacteriophages, also called phages – Have the most complex capsids found among viruses 80  225 nm 50 nm (d) Bacteriophage T4 DNA Head Tail fiber Tail sheath

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure In general, there are three main morphological virus types:

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1- helical These viruses are composed of a single type of capsomere stacked around a central axis to form a helical structure, which may have a central cavity, or hollow tube.

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2-Icosahedral An icosahedron is a convex polyhedron with twenty faces

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3- Complex- usually consists of a head piece and a sheath or tail.

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings General Features of Viral Reproductive Cycles Viruses are obligate intracellular parasites – They can reproduce only within a host cell Each virus has a host range – A limited number of host cells that it can infect

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viruses use enzymes, ribosomes, and small molecules of host cells – To synthesize progeny viruses VIRUS Capsid proteins mRNA Viral DNA HOST CELL Viral DNA DNA Capsid Entry into cell and uncoating of DNA Replication Transcription Self-assembly of new virus particles and their exit from cell

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Phages Phages – Are the best understood of all viruses – Go through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lytic Cycle The lytic cycle (productive) or virulent – Is a phage reproductive cycle that culminates in the death of the host – Produces new phages and digests the host’s cell wall, releasing the progeny viruses

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lysogenic Cycle The lysogenic cycle – Replicates the phage genome without destroying the host Temperate phages – Are capable of using both the lytic and lysogenic cycles of reproduction

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Temperate phages "live in harmony" with their host bacteria in lysogeny. When the phage enters the host cell Some of the bacteria will be lysogenized (lysogenic cycle) Others will be lysed (lytic pathway).

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lytic/lysogenic cycle of a ds DNA phage The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage Bacterial chromosome Phage DNA

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What decides whether lytic or lysogeny take place? factors! 1- amount of nutrients low nutrients favors lysogeny, If host cell doesn’t have enough resources to become a “phage factory”, wait for better times. 2-If host cell is damaged, better to enter lytic mode before the cell dies.

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review of lytic and lysogeny http://highered.mcgraw- hill.com/sites/0072556781/student_view0/chapt er17/animation_quiz_2.html

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lytic cycle of a ds DNA virus The lytic cycle of phage T4, a virulent phage Phage assembly Head Tails Tail fibers Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 1 Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. 2 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. 3 Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 4 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 5

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stages involved in T4 replication (ds DNA) 1- Attachment 2- penetration 3- transcription and translation 4- replication of phage DNA and synthesis of proteins 5-assembly 6-release

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lytic single stranded RNA phages Some important things to consider; 1- common ones are MS2 and QB which infect E.coli. 2- they only infect the F+ strains of E.coli because they attach to the F pilus. 3- large burst size and replication rate 4- Need a special enzyme, RNA dependent RNA polymerase

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Filamentous Phages Filamentous phages are viruses that appear as long thin fibers. These viruses infect bacteria, replicate their DNA and make proteins but do not lyse the host. Examples are the M13 virus and fd virus. The host continues to multiply but do so slowly.

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

29. Filamentous phages adsorb to the tip of F+ pilus of E.coli. The ss DNA enters the bacteria and replicates. The phage protein capsomeres are made but they are stored in the cell membrane where they are not detected. As the DNA leaves through the membrane it gains its protein coat. Carrier cells carry the viral components.

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lytic infection by ss DNA phages The phi X174 virus is capable of lytic infection. It contains a + DNA strand. Not filamentous in shape Can infect non F+ cells.

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction In the process known as transduction – Phages carry bacterial genes from one host cell to another 1 Donor cell Recipient cell A+A+ B+B+ A+A+ A+A+ B–B– A–A– B–B– A+A+ Recombinant cell Crossing over Phage infects bacterial cell that has alleles A + and B + Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. A bacterial DNA fragment (in this case a fragment with the A + allele) may be packaged in a phage capsid. Phage with the A + allele from the donor cell infects a recipient A – B – cell, and crossing over (recombination) between donor DNA (brown) and recipient DNA (green) occurs at two places (dotted lines). The genotype of the resulting recombinant cell (A + B – ) differs from the genotypes of both the donor (A + B + ) and the recipient (A – B – ). 2 3 4 5 Phage DNA A+A+ B+B+