State 4 Properties Conferred By Prophages

13.2 Virus Interactions with Host Cells 331

Table 13.3 Some Properties Conferred by Prophage

Microorganism Medical Importance

Property Coded by Phage

Cotynebacterium diphtheriae

Causes diphtheria

Synthesis of diphtheria toxin

Clostridium botulinum

Causes botulism

Synthesis of botulinum toxin

Streptococcus pyogenes (ß-hemolytic)

Causes scarlet fever

Streptoccal exotoxin responsible for scarlet fever

Salmonella

Causes food poisoning

Modification of lipopolysaccharide of cell wall

Vibrio cholerae

Causes cholera

Synthesis of cholera toxin

long as this repressor is produced, the integrated phage DNA is not excised. If the repressor is no longer synthesized or is inactivated, however, viral genes are transcribed and an enzyme, an excisase which excises the viral DNA from the bacterial chromosome, is synthesized. Vegetative reproduction with lysis ensues (see figure 13.6d). ■ repressor, p. 183 ■ operator, p. 183

Under ordinary conditions of growth, the phage DNA is excised from the chromsome only about once in 10,000 divisions of the lysogen. If, however, a lysogenic culture is treated with an agent that damages the bacterial DNA (such as ultraviolet light), the repair system, called SOS, comes into play. This system activates a protease that destroys the repressor. As a result, all of the prophage enter the lytic cycle, and a productive infection results. In this way the phage escape from a host that is in deep trouble because its DNA has been damaged. This process, termed phage induction, results in complete lysis of the culture. The term induction as it is used here should not be confused with induced enzyme synthesis as discussed in chapter 7 or with phage-induced enzymes discussed earlier in this chapter. ■ SOS system, p. 197 ■ ultraviolet light, p. 123

Immunity of Lysogens

In addition to maintaining the prophage in the integrated state, the repressor protein produced by a prophage prevents the infection of a lysogenic cell by phage of the same type as the phage DNA already carried by the lysogenic cell. Infections are blocked because the repressor binds to the operator in the phage DNA as it enters the cell and inhibits its replication. Consequently, the cell is immune to infection by the same phage but not to infection by other phages, to whose DNA the repressor cannot bind. In this way the phage protects its turf from closely related phages.

Lysogenic Conversion

Lysogenic cells may also differ from their nonlysogenic counterparts in other important ways. The prophage can confer new properties on the cell, the phenomenon of lysogenic conversion. For example, strains of Corynebacterium diphtheriae that are lysogenic for a certain phage (b phage) synthesize the toxin that causes diphtheria. Similarly, lysogenic strains of Streptococcus pyogenes and Clostridium botulinum manufacture toxins that are responsible for scarlet fever and botulism, respectively. In all of these cases, if the prophage is eliminated from the bacterium, the cells lose the ability to synthesize toxin. The genes that code for these toxins are phage genes, which are expressed only when the phage DNA is integrated into the bacterial chromosome. Some properties of bacteria conferred by prophage are given in table 13.3.

Extrusion Following Phage Replication—Filamentous Phages

In addition to the phages that develop productive (lytic) infections and latency, some phage can develop other relationships with their host cells. A few closely related bacterial viruses that appear as long thin fibers are known as the filamentous phages. These include M13 and fd (figure 13.8). They are single-stranded DNA phages that do not take over the metabolism of the host cell for the exclusive production of phage. They do not lyse the cell; rather, these phages are released by extrusion. This process does not destroy the host cell which continues to multiply. Indeed, these few phages are the only ones that do not lyse their hosts, although they make the cells unhealthy.

Replication of Filamentous Phages

In the replication cycle of filamentous phages that are extruded, the phage first adsorbs to the tip of the F+ pilus of E. coli and therefore only infects male cells (figure 13.9). It is clear

Structure Filamentous Phage
Figure 13.8 Electronphotomicrograph of fd Phage This phage only infects cells that have a sex pilus. Each phage is about 900 nm long.

332 Chapter 13 Viruses of Bacteria that the single-stranded DNA that enters the cell does not completely take over their host's metabolism exclusively for phage production because the bacteria continue to multiply. The phage DNA replicates and also codes for the synthesis of the phage coats. Interestingly, neither mature filamentous phage nor their coats can be detected in the cytoplasm of the host cells. It seems likely that after the phage coats are synthesized, they are stored in the cytoplasmic membrane of the bacterium. The phage are assembled as they are extruded from the cell. The extrusion occurs continuously, and the phage are not released in a burst. Infected cells are termed carrier cells. They can be subcultured and stored in the same way as non-infected cells.

Replication of Single-Stranded DNA of Filamentous Phage

The replication of the single-stranded DNA of a filamentous phage has some features that are similar but others that are different from double-stranded DNA replication. The DNA that enters the cell is a positive (+) molecule that is then converted to a double-stranded form by enzymes of the host cell (figure 13.10). The positive (+) strand of DNA is the strand that is not used as a template for mRNA synthesis. The negative (—) strand is used as a template for synthesis of mRNA, which is then translated into phage proteins. The double-stranded, replicative form consists of a positive (+) and a negative (—) strand. The double-stranded replicative form replicates in much the same way as double-stranded DNA of bacteria. This replicative form gives

Phage DNA

Filamentous phage

Cytoplasmic membrane

Cell wall

Coat protein

Phage adsorbs to the tip of the sex pilus of the male cell

Phage DNA

Cytoplasmic membrane \

Figure 13.9 Replication of a Filamentous Phage The phage coat protein is embedded in the cytoplasmic membrane of the bacterium. Maturation of the phage occurs as the DNA is extruded through the cytoplasmic membrane. Carrier cells continue to extrude phage as they multiply.

Filamentous phage

Cytoplasmic membrane

Phage DNA

Cell wall

Phage adsorbs to the tip of the sex pilus of the male cell

Coat protein

Phage DNA

Cytoplasmic membrane \

Phage DNA replicates and phage coat proteins are synthesized and embedded in the cytoplasmic membrane. The phage are released after the nucleic acid gains its protein coat as it passes through the cytoplasmic membrane. The bacteria do not lyse.

Figure 13.9 Replication of a Filamentous Phage The phage coat protein is embedded in the cytoplasmic membrane of the bacterium. Maturation of the phage occurs as the DNA is extruded through the cytoplasmic membrane. Carrier cells continue to extrude phage as they multiply.

Carrier cell

Carrier cell

Carrier cell

Carrier cell

Carrier cell

Carrier cell

Carrier cell

Carrier cells

Carrier cells

Carrier cells

13.3 Transduction 333

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Responses

  • ROBERTO
    What are the properties of prophages.?
    2 years ago
  • Sayid
    What is the properties of lysogenic system?
    12 months ago
  • elsie
    What are d properties dat is conferred by prophage on their respective host?
    10 months ago

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