一、Term
Operon (操纵元)
Cistron顺反子:DNA
Polycistronic message ( 多顺反子 ):RNA
Promoter启动子
Initiaiton site 起始位点
Terminator终止子
Operator ( 操纵区 )
Attenuator弱化子
二、Prokaryotic Gene Structure
1、Operon ( 操纵元): a unit of gene expression in prokarytoes and which includes structural genes and control elements.
Structural gene clusters are coordinately controlled结构基因簇是协同控制的。
Cistron顺反子:A section of DNA that contains the genetic code for a single polypeptide and functions as a hereditary unit. 含有单一多肽遗传密码的DNA片段,起到遗传单位的作用。
Polycistronic message ( 多顺反子 ):an mRNA bearing information from more than one gene
2、Promoter启动子
A DNA sequence to which RNA polymerase binds prior to initiation of transcription-usually located just upstream of the transcription start site of a gene.
启动子的两个核心elements:
- 10 Box (Pribnow box) : TATAAT, centered in -10 position.
- 35 box: TTGACA, centered in -35 bp position.
UP element上游元件: some promoter has an extra element in -40~-60.
3、Initiation site
- The transcription initiation site is a purine in 90% of genes
- 更可能是CGT或者CAT,并且A的可能性大于G
三、The mechanism of transcription in bacteria
1、RNA polymerase
RNA polymerase holoenzyme ( 全酶) includes β’, β, σ & α2,五个subunit
Core enzyme ( 核心酶) : without σsubunit,也就是β’, β, α2四个subunit
Core enzyme only transcribe nicked DNA template, not intact DNA
核心酶只能转录有缺刻的DNA模版,但是完整的DNA无法转录。
Without σ factor, the core enzyme lost specificity of transcription Initiation site。因此σ factor决定了起始位点的特异性。
2、Transcription Initiation
(1)Holoenzyme search for a binding site on the DNA
(2)Forming the closed promoter complex
(3)Bounding tightly, melting DNA and forming an open promoter complex
(4)Polymerase remains at the promoter, in an initial transcribing complex
(5)elongation。sigma stimulate initiation, but not elongation of transcription
σ factor can be reused
σ factor may not dissociate from core during elongation
Two conserved regions of σ are involved in promoter -10 box and -35 box recognition
Summary:(σ factor的作用)
(1) σ factor is involved in promoter -10 box and -35 box recognition, so σ factor can select the transcriptional initiation site.
(2) The σ factor by itself cannot bind to DNA, but interaction with core unmasks a DNA- binding region of σ.
(3) After the initiation, σ factor is bound loosely in the elongating state.
关于结合DNA具体的位点,可以使用DNase footprinting
Summary: functions of α,β, β'subunits of RNA polymerase
(1) All these three subunits are involved in the extension of RNA strain
(2) α subunit can interact with the UP element of promoter to enhance the transcription
(3) β are involved in the formation of phosphodiester bond
(4) β' subunit Stimulates σ binding to core element of promoter
3、Elongation
Need core enzyme
DNA unwinding: topoisomerases
4、Termination
(1)不依赖ρ因子的终止
ρ (rho) independent termination:
- An inverted repeat that allow a hairpin to form at the end of the transcription
- A string of Ts in the non-template strand that results in a string of weak rU-dA base pairs holding the transcription to the template strand.
(2)依赖ρ因子的终止
- Need ρ factors
- Need the inverted repeat that allow a hairpin to form at the end of the transcription
- No need of a string of Ts in the non- template strand
四、原核生物中的转录调控
Each operon is subjected to stringent regulation
This allows the bacteria to be efficient :it will specific proteins only when they are required
Operator ( 操纵区 ): A DNA element found in prokaryotes that binds tightly to a specific repressor and thereby regulates the expression of adjoining genes
(一)Lac operon
三个cistron:(structure genes)lac Z、lac Y、lac A
(二)Ara Operon
Positive control by CAP-cAMP complex, which is similar to Lac operon
(三)The Trp Operon ( 反馈抑制)
1、Trp operon
Genes for enzymes making tryptophan Repression requires the binding of trp and apo-
repressor ( 辅阻遏蛋白 )
Aporepressor is inactive, only when it binds to
tryptophan, act as a repressor.
Low trp, no repression
High trp, repression
2、Attenuator: a region of DNA upstream of one or more structure genes, where premature transcription termination can occur
Attenuator 通过过早终止mRNA合成来调控转录。比如trp操纵子,当色氨酸充足时,trp操纵子被提前终止,色氨酸合成酶的转录被终止。