Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

17 The Viruses: Bacteriophages

CHAPTER OVERVIEW

This chapter focuses on the characteristics of the bacterial viruses, or bacteriophages. It begins with their classification and then details the infectious cycle of those DNA viruses that cause destruction (lysis) of host cells. RNA phages are discussed briefly, and the chapter concludes with information about phages that can integrate their DNA into the host chromosome and thereby set up a stable residence within the host cell. These phages are called temperate phages, and the process is referred to as lysogeny.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

o describe the four phases of the viral life cycle

o discuss the differences between DNA phages and RNA phages in terms of their life cycles and their interactions with their hosts

o discuss the establishment and maintenance of lysogeny by temperate phages

CHAPTER OUTLINE

I. Classification of Bacteriophages-the most important criteria are phage morphology and nucleic acid properties

A. Morphology

1. Tailless icosahedral

2. Viruses with contractile tails

3. Viruses with noncontractile tails

4. Filamentous viruses

B. Nucleic acid properties

1. DNA or RNA

2. Single stranded (ss) or double stranded (ds)

II. Reproduction of DNA Phages: The Lytic Cycle-culminates with the host cell bursting and releasing virions

A. The one-step growth experiment

1. Reproduction is synchronized

2. Bacteria are infected and then diluted so that the released phages will not immediately find new cells to infect

3. The released phages are then enumerated

4. Several distinct phases are observed

a. Latent period-no release of virions detected; represents the shortest time required for virus reproduction and release; the early part of this period is called the eclipse period, and during this period no infective virions can be found even inside infected cells

b. Rise period (burst)-rapid lysis of host cells and release of infective phages

c. Plateau period-no further release of infective virions

5. Burst size-the number of infective virions released per infected cell

B. Adsorption to the host cell and penetration

1. Viruses attach to specific receptor sites (proteins, lipopolysaccharides, teichoic acids, etc.)

2. Receptor variation is at least partly responsible for host range specificity

3. Binding is probably due to electrostatic interactions and is influenced by pH and the presence of ions such as Mg²⁺ and Ca²⁺

4. Many viruses inject DNA into the host cell, leaving an empty capsid outside

5. The tail tube of T-even bacteriophages may interact with the plasma membrane to form a pore through which the DNA passes

C. Synthesis of phage nucleic acids and proteins

1. Host synthesis of DNA, RNA, and protein is halted

2. Some virus-specific mRNAs (early mRNA) are transcribed using host RNA polymerase

3. Early proteins, made at the direction of these mRNAs, may:

a. Take over host cells

b. Degrade host DNA

c. Replicate the viral nucleic acid

d. Alter host RNA polymerase to preferentially transcribe viral genes by altering promoter recognition

4. Viral DNA is replicated

a. Alternate bases (if needed) are synthesized; these are sometimes used to protect the phage DNA from host enzymes (restriction endonucleases) that would otherwise degrade the viral DNA and thereby protect the host

b. Replication of viral DNA-the mechanism of single-stranded DNA phage replication is different from that of double-stranded DNA phage replication; single-stranded DNA phages usually require the formation of a double-stranded replicative form (RF), which in turn directs the synthesis of mRNA and the new genome

D. The Assembly of phage particles

1. Capsid proteins are synthesized at the direction of late mRNAs (made after viral nucleic acid replication)

2. Noncapsid (scaffolding) proteins needed for assembly and/or lysis are also made at the direction of late mRNAs

3. Assembly proceeds either sequentially or by subassemblies, which are then put together

4. Different phages package viral DNA differently

a. Some phages build capsid around the DNA

b. Some phages insert DNA into preformed (but as yet incomplete) capsid structures

E. Release of phage particles

1. Enzymes damage the cytoplasmic membrane

2. Other enzymes damage the cell wall

3. A few phages (e.g., filamentous fd phages) are released without lysing the host cell, which secretes the phage through the plasma membrane (into which phage coat proteins have been inserted)

III. Reproduction of RNA Phages

A. RNA replicase-the virus must provide an enzyme for replicating the RNA genome because the host does not produce an enzyme with this capability

B. RNA is usually plus stranded (+) and can act similarly to mRNA in directing the synthesis of the replicase during an initial step after penetration

C. +ssRNA is then converted to ądsRNA, the replicative form

D. Replicative form is then used as a template for production of multiple copies of the genomic (and messenger) +ssRNA

E. Capsid proteins are made, and +ssRNA is packaged into new virions

F. One or more lysis proteins then function to release the phage

G. Only one dsRNA phage has so far been discovered (f6)

1. It infects Pseudomonas phaseolicola

2. It possesses a membranous envelope

IV. Temperate Bacteriophages and Lysogeny

A. Temperate phages are capable of lysogeny, a nonlytic relationship with their hosts

B. In lysogeny, the viral genome (called a prophage) is integrated into the host DNA and is replicated with it; it does not kill (lyse) the host cell; the cells are said to be lysogenic (or are called lysogens)

C. In the lysogenic state, the host cell cannot be superinfected by a virus of the same type (i.e., it has immunity to superinfection)

D. It may switch to the lytic cycle at some later time; this process is called induction

E. Conditions at the time of infection may determine whether the virus will establish a lytic infection or lysogeny

F. Establishment of lysogeny (bacteriophage lambda)

1. Two sets of promoters are available to host RNA polymerase

2. A repressor protein may be made from genes adjacent to one of these promoters

3. If this repressor binds to its target operator before the other promoter is used, then that promoter is blocked and lysogeny is established

4. If genes associated with that second promoter are expressed before the repressor can bind to the operator, then the lytic cycle is established

5. For lambda and most temperate phages, if lysogeny is established, the viral genome integrates into the host chromosome; however, some temperate phages can establish lysogeny without integration

6. Induction (the termination of lysogeny and entry into the lytic cycle) will occur if the level of the repressor protein decreases; this is usually in response to environmental damage to the host DNA

G. Lysogenic conversion is a change that is induced in the host phenotype by the presence of a prophage, and that is not directly related to the completion of the viral life cycle; examples include:

1. Modification of lipopolysaccharide structure in infected Salmonella

2. Production of diphtheria toxin only by lysogenized strains of Corynebacterium diphtheriae

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