m 395 Bg的 BP Lysogenic Figure 17.20 Reversible Insertion and Exeision of Lambda Pha After circularization the att site pp and in eny.The Ecoli phage PI is similar to lambda in thatit How is a pre age induced to bccome active again? d at the of the phage genome. Prescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 binds to the integrase and enables it to reverse the integration process and free the prophage (figure 17.20). The lytic cycle then proceeds normally. Most temperate phages exist as integrated prophages in the lysogen. Nevertheless, integration is not an absolute requirement for lysogeny. The E. coli phage P1 is similar to lambda in that it circularizes after infection and begins to manufacture repressor. However, it remains as an independent circular DNA molecule in the lysogen and is replicated at the same time as the host chro￾mosome. When E. coli divides, P1 DNA is apportioned between the daughter cells so that all lysogens contain one or two copies of the phage genome. 1. Define virulent phage, lysogeny, temperate phage, lysogen, prophage, immunity, and induction. 2. What advantages might a phage gain by being capable of lysogeny? 3. Describe lysogenic conversion and its significance. 4. Precisely how, in molecular terms, is a bacterial cell made lysogenic by a temperate phage like lambda? 5. How is a prophage induced to become active again? 6. Be able to describe the roles of the lambda repressor, cro protein, the recA protein, integrase, and excisionase in lysogeny and induction. 7. How does the temperate phage P1 differ from lambda phage? 17.5 Temperate Bacteriophages and Lysogeny 395 J N (d) Prophage gal B P′ N R m′ m A J P B′ bio Lysogenic chromosome (c) B gal P′ m′ m A R J N P B′ bio (b) gal B B′ bio Bacterial chromosome (a) P P′ att A m m′ R λ chromosome Integrase Figure 17.20 Reversible Insertion and Excision of Lambda Phage. After circularization,the att site P, P′ (a) lines up with a corresponding bacterial sequence B, B′ (b) and is integrated between the gal and bio operons to form the prophage, (c) and (d). If the process is reversed, the circular lambda chromosome will be restored and can then reproduce