Chapter 8 Major Shifts in Prokaryotic Transcription Copyright The McGraw-Hill Companies,Inc.Permission reg production or disple ers
Chapter 8 Major Shifts in Prokaryotic Transcription
When a phage infects a bacterium,it usually subvert the host's transcription machinery to its own use.the early phage genes->the later genes. This massive shift in specificity would be hard to explain by the operon mechanisms.Instead,it is engineered by a fundamental change in the transcription machinery-a change in RNA polymerase itself. the most common of these changes in RNA polymerase are actually changes in the o-factor, which governs the specificity of transcription
✓When a phage infects a bacterium, it usually subvert the host’s transcription machinery to its own use. the early phage genes → the later genes. ✓This massive shift in specificity would be hard to explain by the operon mechanisms. Instead, it is engineered by a fundamental change in the transcription machinery-a change in RNA polymerase itself. ✓the most common of these changes in RNA polymerase are actually changes in the σ–factor, which governs the specificity of transcription
8.1 Modification of the Host RNA Polymerase During Phage Infection
8.1 Modification of the Host RNA Polymerase During Phage Infection
(a)Early transcription:specificity factor:host() transcription: 200D0 Early genes the early Host RNA polymerase holoenzyme Early transcripts genes are expressed Early proteins,.including gp28(○） (b)Middle transcription;specificity factor:gp28() the gp28,associates with the host middle Middle genes core polymerase genes Middle transcripts turn Middle proteins,including gp33()and gp34() gp33 and gp34(as o-factor) the late replace gp28 and direct (c)Late transcription;specificity factor:gp33()+gp34() genes the altered polymerase to switch transcribe the phage late 0n. Late genes genes Late transcripts- →Late proteins
the late genes switch on. transcription: the early genes are expressed the middle genes turn Host RNA polymerase holoenzyme gp28, associates with the host core polymerase gp33 and gp34(as σ-factor) replace gp28 and direct the altered polymerase to transcribe the phage late genes
the polypeptides of the host core polymerase remain constant throughout this process;it is the progressive substitution of o-factors that changes the specificity of the enzyme and thereby directs the transcriptional program. the changes in transcription specificity also depend on the fact that the early,middle,and late genes have promoters with different sequences.That is how they can be recognized by different o-factors
✓the polypeptides of the host core polymerase remain constant throughout this process; it is the progressive substitution of σ-factors that changes the specificity of the enzyme and thereby directs the transcriptional program. ✓ the changes in transcription specificity also depend on the fact that the early, middle, and late genes have promoters with different sequences. That is how they can be recognized by different σ-factors
小结 √噬菌体SPO1感染枯草芽孢杆菌时，其基因转录呈 时序性进行：首先是早期基因，然后是中期基因， 最后是晚期基因。 √这种转换是由噬菌体所编码的一组σ因子来控制。 这些σ因子通过与宿主的核心RNA聚合酶结合， 改变RNA聚合酶对早、中、晚期基因启动子识别 的特异性。 √宿主的σ因子对噬菌体早期基因具有特异性；而 噬菌体的g即28蛋白将这种特异性转换为针对中期 基因；噬菌体的g33和34蛋白又将特异性转换为 针对晚期基因
小结 ✓噬菌体SPO1感染枯草芽孢杆菌时,其基因转录呈 时序性进行：首先是早期基因，然后是中期基因， 最后是晚期基因。 ✓这种转换是由噬菌体所编码的一组σ因子来控制。 这些σ因子通过与宿主的核心RNA聚合酶结合， 改变RNA聚合酶对早、中、晚期基因启动子识别 的特异性。 ✓宿主的σ因子对噬菌体早期基因具有特异性；而 噬菌体的gp28蛋白将这种特异性转换为针对中期 基因；噬菌体的gp33和34蛋白又将特异性转换为 针对晚期基因
8.2 The RNA Ploymerase Encode in PhageT7 In these phases we distinguish three phases of transcription,called classes I,II, and III.(They could just as easily be called early,middle,and late,to conform to SPO1 nomenclature.) One of the five class I genes (genel)is necessary for class II and class III gene expression,The gene 1 product is a phage- specific RNA polymerase
8.2 The RNA Ploymerase Encode in PhageT7 ✓In these phases we distinguish three phases of transcription, called classes I, II, and III. (They could just as easily be called early, middle, and late, to conform to SPO1 nomenclature.) ✓One of the five class I genes (gene1) is necessary for class II and class III gene expression,The gene 1 product is a phagespecific RNA polymerase
(a)Early transcription;specificity factor:host o() The switching mechanism in T7 Class I genes When the phage DNA enter the host Class I transcripts cell,the E.coli holoenzyme transcribes the five class I genes,including gene 1 Class I proteins,including phage RNA polymerase() (b)Late transcription;phage RNA polymerase The gene 1 product(phage-specific RNA polymerase)then transcribes the phage class II and class III genes. Class ll and Ill genes Class ll and Ill transcripts Class ll and lll proteins
The gene 1 product (phage-specific RNA polymerase)then transcribes the phage class II and class III genes. When the phage DNA enter the host cell, the E.coli holoenzyme transcribes the five class I genes, including gene 1 The switching mechanism in T7
小结 √噬菌体T7编码一个新的对噬菌体晚期基因具 有绝对特异性的RNA聚合酶。这个由一条多 肽组成的聚合酶是噬菌体最早的基因之一 (基因1)的产物。 √这种噬菌体感染细胞后其转录过程较为简单。 宿主聚合酶转录早期基因（型基因)，其产 物之一是噬菌体聚合酶，该酶转录晚期基因 (I和Ⅲ型基因)
小结 ✓噬菌体T7编码一个新的对噬菌体晚期基因具 有绝对特异性的RNA聚合酶。这个由一条多 肽组成的聚合酶是噬菌体最早的基因之一 （基因1）的产物。 ✓这种噬菌体感染细胞后其转录过程较为简单。 宿主聚合酶转录早期基因（I型基因），其产 物之一是噬菌体聚合酶，该酶转录晚期基因 （II和III型基因）
8.3 Control of Transcription During Sporulation (孢子形成) in the vegetative,or growth, under starvation or other state,as long as nutrients are adverse conditions,this available and other conditions organism forms are appropriate for growth. endospores How is the transcription-switching mechanism able to cope with these complexities?
8.3 Control of Transcription During Sporulation （孢子形成） under starvation or other adverse conditions, this organism forms endospores in the vegetative, or growth, state, as long as nutrients are available and other conditions are appropriate for growth. How is the transcription-switching mechanism able to cope with these complexities?