94 Chapter 17 The Viruses:Bacteriophoge (b) epressor (gpel)is rapidly synthesized and binds to and ase enzyme.and this protein becomes plentiful before synthesis and the production of the cland lambda repressor turs off transcription.Lambda DNA has a 、the repe.thisentvated by tw sures that lysogenywll in. )The circular lambda DNA is inte vould be established cro proteir ar order of phage genes has been changed or permuted nes) he will leave theE.nme and be I8d and figure 17 9 The d repress repress ally the re sor wil nduction usually is in rest such a ore re UV light or che DNA rion )to act as a umber and clea The bleeen the tween the lytic and mal ac DNA tive again. d recPrescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 repressor (gpcI) is rapidly synthesized and binds to OR and OL, thus turning off mRNA synthesis and the production of the cII and cIII proteins (figure 17.18c). The cI gene continues to be tran￾scribed at a low rate because of the activity of a second promoter (PRM, RM stands for repressor maintenance) that is activated by the repressor itself. This control circuit in which lambda repressor stimulates its own synthesis ensures that lysogeny will normally be stable when once established. One might expect that lysogeny would be established every time but this is not the case. During this period the cro protein (gpcro) has also been accumulating. The cro protein binds to OR and OL, turns off the transcription of the repressor gene (as well as inhibiting the expression of other early genes), and represses PRM function (figure 17.18d and figure 17.19). Because the lambda repressor can block cro transcription, there is a race between the production of lambda repressor and that of the cro protein. Although cro protein synthesis begins before that of the lambda repressor, gpcro binds to OR more weakly and must rise to a higher level than the repressor before re￾pressor synthesis is blocked and the lytic cycle started (figure 17.18d). The details of this competition are not yet completely clear, but it has been shown that a number of environmental factors influence the outcome of the race and the choice be￾tween the lytic and lysogenic pathways. If the lambda repressor wins the race, the circular lambda DNA is inserted into the E. coli genome as first proposed by Alan Campbell. Integration or insertion is possible because the cII protein stimulates transcription of the int gene at the same time as that of the cI gene. The int gene codes for the synthesis of an in￾tegrase enzyme, and this protein becomes plentiful before lambda repressor turns off transcription. Lambda DNA has a phage attachment site (the att site) that can base pair with a bac￾terial attachment site located between the galactose or gal operon and the biotin operon on the E. coli chromosome. After these two sites match up, the integrase enzyme, with the aid of a special host protein, catalyzes the physical exchange of viral and bacterial DNA strands (figure 17.20). The circular lambda DNA is inte￾grated into the E. coli DNA as a linear region next to the gal operon and is called a prophage. As can be seen in figure 17.20, the linear order of phage genes has been changed or permuted during integration. The lambda prophage will leave the E. coli genome and be￾gin the production of new phages when the host is unable to sur￾vive. The process is known as induction and is triggered by a drop in lambda repressor levels. Occasionally the repressor will spontaneously decline and the lytic cycle commence. However, induction usually is in response to environmental factors such as UV light or chemical mutagens that damage host DNA. This damage causes the recA protein, which normally plays a role in genetic recombination in E. coli (see section 11.8), to act as a protease and cleave the repressor chain between the two do￾mains. The separated domains cannot assemble to form the nor￾mal active repressor dimer, and the lytic cycle genes become ac￾tive again. There is some recent evidence that activated recA protein may not directly cleave the repressor. RecA may instead bind to the lambda repressor and stimulate it to proteolytically cleave itself. An early gene located next to the int gene, the xis gene, codes for the synthesis of an excisionase protein that 394 Chapter 17 The Viruses: Bacteriophages 17 bp 1 1 2 2 3 3 (a) (b) Figure 17.19 Cro Protein Binding. (a) A space-filling model of the cro protein–DNA complex. The cro protein is in yellow. (b) A diagram of the cro protein dimer–DNA complex. Like the lambda repressor protein, the cro protein functions as a dimer and binds to two adjacent DNA major grooves