90 Chapter 17 The Viruses: (mRNA activity 17.5 Temperate Bacteriophages and Lysogeny Up to this point many of the viruses we have discu lent bacteriophages:thes remains within the host cell and repli ates with the bacterial cannot.for conditions The kepr on of Single-Stranded RN sby which phag oduction is initiat uon of (figure 17.13).The replicase then copies the original RNA (a e.Lys of tran pitstrmndtoprotceado ination.but v DNA seen in the reproduction of ssDNA p The sam have been studied are temp =RNA in order to accelerate +RNA synthesis.Other aen网 virus particles.The genome of these RNA phages serves as both eis faced with two problems:it can only rep lizing bacd by wh ich have becn n This predicament can be a becomes dorman e as its ho wn. e in situation nan cell initiate maturation).The (MOD.Whe will destroy all h st cells.Thus there is a risk that the phages may d,the bacteric The )a and thre ves alter s in b e charac 1.How are ssRNA phages reproduced,and what role does RNA phage. structure of its oute )may be TedhcPrescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 (figure 17.13). The replicase then copies the original RNA (a plus strand) to produce a double-stranded intermediate (RNA), which is called the replicative form and is analogous to the DNA seen in the reproduction of ssDNA phages. The same replicase next uses this replicative form to synthesize thousands of copies of RNA. Some of these plus strands are used to make more RNA in order to accelerate RNA synthesis. Other RNA acts as mRNA and directs the synthesis of phage pro￾teins. Finally, RNA strands are incorporated into maturing virus particles. The genome of these RNA phages serves as both a template for its own replication and an mRNA. MS2 and Q, family Leviviridae, are small, tailless, icosa￾hedral ssRNA phages of E. coli, which have been intensely studied (figure 17.1). They attach to the F-pili of their host and enter by an unknown mechanism. These phages have only three or four genes and are genetically the simplest phages known. In MS2, one protein is involved in phage adsorption to the host cell (and possibly also in virion construction or maturation). The other three genes code for a coat protein, an RNA replicase, and a protein needed for cell lysis. Only one dsRNA phage has been discovered, the bacterio￾phage 6 of Pseudomonas phaseolicola (figure 17.1). It is also unusual in possessing a membranous envelope. The icosahedral capsid within its envelope contains an RNA polymerase and three dsRNA segments, each of which directs the synthesis of an mRNA. It is not yet known how the dsRNAs are replicated. 1. How are ssRNA phages reproduced, and what role does RNA replicase play in the process? 2. What is peculiar about the structure of phage 6? 17.5 Temperate Bacteriophages and Lysogeny Up to this point many of the viruses we have discussed are viru￾lent bacteriophages; these are phages that lyse their host cells during the reproductive cycle. Many DNA phages also can estab￾lish a different relationship with their host. After adsorption and penetration, the viral genome does not take control of its host and destroy it while producing new phages. Instead the viral genome remains within the host cell and replicates with the bacterial genome to generate a clone of infected cells that may grow and di￾vide for long periods while appearing perfectly normal (see figure 13.18). Each of these infected bacteria can produce phages and lyse under appropriate environmental conditions. They cannot, for reasons that will become clear later, be reinfected by the same virus—that is, they have immunity to superinfection. This rela￾tionship between the phage and its host is called lysogeny. Bacte￾ria having the potential to produce phage particles under some conditions are said to be lysogens or lysogenic, and phages able to enter into this relationship are temperate phages. The latent form of the virus genome that remains within the host but does not de￾stroy it is called the prophage. The prophage usually is integrated into the bacterial genome but sometimes exists independently. In￾duction is the process by which phage reproduction is initiated in a lysogenized culture. It leads to the destruction of infected cells and the release of new phages—that is, induction of the lytic cy￾cle. Lysogeny was briefly described earlier in the context of trans￾duction and genetic recombination, but will be discussed in more detail here. Generalized and specialized transduction (pp. 307–9) Most bacteriophages that have been studied are temperate, and it appears that there are advantages in being able to lysoge￾nize bacteria. Consider a phage-infected culture that is becoming dormant due to nutrient deprivation. Before bacteria enter dor￾mancy, they degrade their own mRNA and protein. Thus the phage is faced with two problems: it can only reproduce in ac￾tively metabolizing bacteria, and phage reproduction is usually permanently interrupted by the mRNA and protein degradation. This predicament can be avoided if the phage becomes dormant (lysogenic) at the same time as its host; in fact, nutrient depriva￾tion does favor lysogeny. Temperate phages also have an advan￾tage in situations where many viruses per cell initiate an infec￾tion—that is, where there is a high multiplicity of infection (MOI). When every cell is infected, the last round of replication will destroy all host cells. Thus there is a risk that the phages may be left without a host and directly exposed to environmental haz￾ards for months or years. This prospect is avoided if lysogeny is favored by a high MOI; some bacteria will survive, carry the virus genome, and synthesize new copies as they reproduce. Not sur￾prisingly a high MOI does stimulate lysogeny. A temperate phage may induce a change in the phenotype of its host cell that is not directly related to completion of its life cy￾cle. Such a change is called a lysogenic conversion or a conver￾sion and often involves alterations in bacterial surface character￾istics or pathogenic properties. For example, when Salmonella is infected by an epsilon phage, the structure of its outer lipopolysaccharide layer (see pp. 58–60) may be modified. The phage changes the activities of several enzymes involved in con- 390 Chapter 17 The Viruses: Bacteriophages + RNA RNA replicase (mRNA activity) ± RNA (replicative form) Replication RNA replicase + RNA mRNA activity Virus proteins + RNA genomes New virions Translation ( ( Figure 17.13 The Reproduction of Single-Stranded RNA Bacteriophages.