IVL.The Viruse n 8e 391 regul example is the ter age B of tion,DNAr on.and cell lysis. th diphiheriae that PR) nd a le e(PL mperate phag gin bothdirc ons,copying different DNA or randed stretches 12 nucleotides lone that have time seque and will be manufactured only when needed dur re similar to the same pr esses already described for the T4phage.One sig nicanfcremcehouldbecnotecd are seen (see section).lambda DNA is primarily synthe fgue11.12). end(gue17.17】 most active fomm of the sor).In a lysogen the re and Figure 17.14 Bacteriophage Lambda. poibehcecdiouhriati -3 neir complementary base sequences Prescott−Harley−Klein: Microbiology, Fifth Edition VI. The Viruses 17. The Viruses: Bacteriophages © The McGraw−Hill Companies, 2002 3′ 5′ P 3′ GGGCGGCGACCT P 5′ Circularization CCCGCCGCTGGA Open circle T A C G struction of the lipopolysaccharide carbohydrate component and thus alters the antigenic properties of the host. These epsilon￾induced changes appear to eliminate surface phage receptors and prevent infection of the lysogen by another epsilon phage. An￾other example is the temperate phage  of Corynebacterium diphtheriae, the cause of diphtheria. Only C. diphtheriae that is lysogenized with phage  will produce diphtheria toxin (see sec￾tions 34.3 and 39.1) because the phage, not the bacterium, carries the toxin gene. The lambda phage, family Siphoviridae, that uses the K12 strain of E. coli as its host is the best-understood temperate phage and will serve as our example of lysogeny. Lambda is a double￾stranded DNA phage possessing an icosahedral head 55 nm in diameter and a noncontractile tail with a thin tail fiber at its end (fig￾ure 17.14). The DNA is a linear molecule with cohesive ends— single-stranded stretches, 12 nucleotides long, that have comple￾mentary base sequences and can base pair with each other. Because of these cohesive ends, the linear genome cyclizes immediately upon infection (figure 17.15). E. coli DNA ligase then seals the breaks, forming a closed circle. The lambda genome has been carefully mapped, and over 40 genes have been located (figure 17.16). Most genes are clustered according to their function, with separate groups involved in head synthesis, tail synthesis, lysogeny and its regula￾tion, DNA replication, and cell lysis. DNA ligase (p. 239) Lambda phage can reproduce using a normal lytic cycle. Im￾mediately after lambda DNA enters E. coli, it is converted to a co￾valent circle, and transcription by the host RNA polymerase is initiated. As shown in figure 17.16, the polymerase binds to both a rightward promoter (PR) and a leftward promoter (PL) and be￾gins to transcribe in both directions, copying different DNA strands. The first genes that are transcribed code for regulatory proteins that control the lytic cycle: leftward gene N and right￾ward genes cro and cII (figure 17.16). These and other regulatory genes ensure that virus proteins will be synthesized in an orderly time sequence and will be manufactured only when needed dur￾ing the life cycle. Regulation of transcription (pp. 275–78) Lambda DNA replication and virion assembly are similar to the same processes already described for the T4 phage. One sig￾nificant difference should be noted. Although initially bidirec￾tional DNA replication is used and theta-shaped intermediates are seen (see section 11.3), lambda DNA is primarily synthe￾sized by way of the rolling-circle mechanism to form long con￾catemers that are finally cleaved to give complete genomes (see figure 11.12). The establishment of lysogeny and the earlier-mentioned im￾munity of lysogens to superinfection can be accounted for by the presence of the lambda repressor coded for by the cI gene. The re￾pressor protein chain is 236 amino acids long and folds into a dumbbell shape with globular domains at each end (figure 17.17). One domain is concerned with binding to DNA, while the other binds with another repressor molecule to generate a dimer (the most active form of the lambda repressor). In a lysogen the repres￾sor is synthesized continuously and binds to the operators OL and OR, thereby blocking RNA polymerase activity (figure 17.18c). If another lambda phage tries to infect the cell, its mRNA synthesis 17.5 Temperate Bacteriophages and Lysogeny 391 Figure 17.14 Bacteriophage Lambda. Figure 17.15 Lambda Phage DNA. A diagram of lambda phage DNA showing its 12 base, single-stranded cohesive ends (printed in red) and the circularization their complementary base sequences make possible