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