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本课件为《分子生物学》第11章噬菌体策略（Phage strategies），依据 Gene VII 教材编排，沿Lytic development等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 11 Phage strategies
Lytic infection of bacteria by a phage ends in destruction of bacteria and release of progeny phage.
Figure 11.1 Lytic development involves the reproduction of phage particles with destruction of the host bacterium, but lysogenic existence allows the phage genome to be carried as part of the bacterial genetic information.
Figure 11.2 Several types of independent genetic units exist in bacteria.
Figure 11.3 Lytic development takes place by producing phage genomes and protein particles that are assembled into progeny phages.
Figure 11.4 Phage lytic development proceeds by a regulatory cascade, in which a gene product at each stage is needed for expression of the genes at the next stage.
Figure 9.31 Switches in transcriptional specificity can be controlled at initiation or termination.
Figure 11.5 Phage T7 contains three classes of genes that are expressed sequentially. The genome is ~38 kb.
Essential genes are indicated by numbers. Nonessential genes are identified by letters. Only some representative T4 genes are shown on the map.
11.3 Functional clustering in phages T7 and T4 Figure 11.6 The map of T4 is circular. There is extensive clustering of genes coding for components of the phage and processes such as DNA replication, but there is also dispersi on of genes coding for a variety of enzymatic and other functions.
Figure 11.7 The phage T4 lytic cascade falls into two parts: early and quasi-late functions are concerned with DNA synthesis and gene expression; late functions are concerned with particle assembly.
Figure 11.24 RNA polymerase binds to PRE only in the presence of CII, which contacts the region around -35.
Figure 11.8 The lambda lytic cascade is interlocked with the circuitry for lysogeny.
Figure 11.9 The lambda map shows clustering of related functions. The genome is 48,514 bp.
Figure 11.10 Phage lambda has two early transcription units; in the "leftward" unit, the "upper" strand is transcribed toward the left; in the "rightward" unit, the "lower" strand is transcribed toward the right. Promoters are indicated by the shaded red or blue arrowheads. Terminators are indicated by the shaded green boxes. Genes Nand cro are the immediate early functions, and are separated from the delayed early genes by the terminators. Synthesis of N protein allows RNA polymerase to pass the terminators tL1to the left and tR1to the right.
Figure 11.11 Lambda DNA circularizes during infection, so that the late gene cluster is intact in one transcription unit.
Figure 11.12 The lambda regulatory region contains a cluster of trans-acting functions and cis-acting elements.
Figure 11.13 Wild-type and virulent lambda mutants can be distinguished by their plaque types. Photograph kindly provided by Dale Kaiser.
Figure 11.14 Lysogeny is maintained by an autogenous circuit (upper). If this circuit is interrupted, the lytic cycle starts (lower).
Figure 11.14 Lysogeny is maintained by an autogenous circuit (upper). If this circuit is interrupted, the lytic cycle starts (lower).
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Figure 11.15 The N-terminal and C-terminal regions of repressor form separate domains. The C-terminal domains associate to form dimers; the N-terminal domains bind DNA.
Figure 11.16 Repressor dimers bind to the operator. The affinity of the N-terminal domains for DNA is controlled by the dimerization of the C-terminal domains.
Figure 11.16 -2 Lambda repressor binds to operators with second-order kinetics.
Figure 11.17 Lambda repressor's N-terminal domain contains five stretches of a-helix; helices 2 and 3 are involved in binding DNA.
Figure 11.18 In the two-helix model for DNA binding, helix-3 of each monomer lies in the wide groove on the same face of DNA, and helix-2 lies across the groove.
Figure 11.19 Two proteins that use the two-helix arrangement to contact DNA recognize lambda operators with affinities determined by the amino acid sequence of helix-3.
Figure 11.20 A view from the back shows that the bulk of the repressor contacts one face of DNA, but its N-terminal arms reach around to the other face.
Figure 11.21 Each operator contains three repressor-binding sites, and overlaps with the promoter at which RNA polymerase binds. The orientation of OL has been reversed from usual to facilitate comparison with OR.
Figure 11.21 Each operator contains three repressor-binding sites, and overlaps with the promoter at which RNA polymerase binds. The orientation of OL has been reversed from usual to facilitate comparison with OR.
说明：本页源文与上一页完全相同，疑为课件连续两页沿用同一页内容，已按原文保留。
Figure 11.14 Lysogeny is maintained by an autogenous circuit (upper). If this circuit is interrupted, the lytic cycle starts (lower).
Figure 11.22 Positive control mutations identify a small region at helix-2 that interacts directly with RNA polymerase.
Figure 11.12 The lambda regulatory region contains a cluster of trans-acting functions and cis-acting elements.11.8 How is repressor synthesis established?.
Figure 11.23 Repressor synthesis is established by the action of CII and RNA polymerase at PRE to initiate transcription that extends from the antisense strand of cro through the cI gene.11.8 How is repressor synthesis established?.
Figure 11.24 RNA polymerase binds to PRE only in the presence of CII, which contacts the region around -35.11.8 How is repressor synthesis established?.
Figure 11.25 Positive regulation can influence RNA polymerase at either stage of initiating transcription.11.8 How is repressor synthesis established?.
Figure 11.26 A cascade is needed to establish lysogeny, but then this circuit is switched off and replaced by the autogenous repressor-maintenance circuit.11.8 How is repressor synthesis established?.
Figure 11.19 Two proteins that use the two-helix arrangement to contact DNA recognize lambda operators with affinities determined by the amino acid sequence of helix-3.
Figure 11.27 The lytic cascade requires Cro protein, which directly prevents repressor maintenance via PRM, as well as turning off delayed early gene expression, indirectly preventing repressor establishment.
Figure 11.26 A cascade is needed to establish lysogeny, but then this circuit is switched off and replaced by the autogenous repressor-maintenance circuit.
1.Phages have a lytic life cycle, in which infection of a host bacterium is followed by production of a large number of phage particles, lysis of the cell, and release of the viruses. 2.Lytic infection falls typically into three phases. In the first phase a small number of phage genes are transcribed by the host RNA polymerase.3.In phage lambda, the genes are organized into groups whose expression is controlled by individual regulatory events.
Summary
4.Each operator consists of three binding sites for repressor. 5.The helix-turn-helix motif is used by other DNA-binding proteins, including lambda Cro, which binds to the same operators, but has a different affinity for the individual operator sites, determined by the sequence of helix-3.6.Establishment of repressor synthesis requires use of the promoter PRE, which is activated by the product of the cII gene.
Summary
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