• G/C rich region that forms hairpins of varying lengths (7-20bp) in RNA
• G/C rich hairpin are followed by a run of U residues in RNA
Function
• G/C rich → transcription delay
4.3 Transcription Elongation 4.3.1 Transcription delay(转录过程的延宕dang）
l 模板DNA中G/C的存在
delay
l promoter clearance
elongation complex
TFII E, H, ATP, Negative-superhelix needed
● Rho factor be needed for termination
Function RNApol + NusA-protein
RNApol pausing 60’’ more or less
K, Rho
termination
or read-through
Rho-factor (ρ)
l Hexamer 55kd neutral l Contest RNA 3’-end with βfactor
被激活的基因的表达方式，其RNA能顺利延伸
RNApol II stop at 3’-end of gene 并表现为DRBS (5,6-2氯-1-β-D呋喃核糖苯并咪唑) (5,6-dichloro-1-β-D ribofuranosyl benzimidazole)
l 非进行型复合体（Non-processive complex）
l 3’-end 具有高亲和力结合的核蛋白质影响 l Silencer or DNA loop 结构的影响 l DNA分子的弯曲
形成系列 调节机制
以降低非生理性，非特异性的基因表达
延伸过程与两种transcription Complex 性质相关
l 进行型复合体 （processive complex）
the end of the transcript, that causes RNA polymerase to terminate transcription.
Rho-independent terminator (Intrinsic
terminator)
Structure
• Palindromic sequence
与 Rho(ρ)因子竞争RNA 3’-end 构成Holo Enzyme后，β因子含有 两个位点
I site (Rif S) ; E site (Rif R) ;
专一性地结合ATP or GTP 要求高浓度的ATP or GTP
对 NTP 非专一性结合
β’ 因子： 强碱性亚基
促使RNA pol与非模板链（sense strand）强结合
l ATPase-like, GTPase-like activity needs spacer of > 50 Nt
l 转录终止效率取决于紧随回文的下游序列 ---保证 RNA / DNA 具有一定松弛的结合力 ---过低的结合力→ 成为 Rho-independent terminator ---过高的结合力→ 引起 RNApol read-through ---各依赖 Rho 因子的终止子间的效率差异取决于 终止子区DNA / RNA 间结合力的差异
c) Rho helicase unwinds the RNA-DNA hybrid and releases the transcript.
受K酶抑制
Rho-dependent terminator
Structure l Palindromic sequence
l no G/C rich in palindrom but form loose hairpin in RNA
l palindromic sequence are not followed by poly A/T in DNA
未被激活的基因的表达方式 RNApol II pausing at 5’-end of gene，并表现为DRBR
--- 是对基因非激活转录的中止机制 --- 维持正常生理代谢进行的机制 --- 与转录启动复合体的非完整性有关（abortive TIC）
e.g. Drosophila hsp70 gene 非热激条件下，RNA pausing at +14 or +42
l RNApol II + TFII F（helicase）
Transcription
After 100 Nt
55kd TFII J stimulation elongation
4.3.2 Pre-mature termination of transcription
—— RNA pausing
由于核小体致密的结构
• RNA stem-loop & polyA/U →RNApol leave
• β’ inhibited by K factor → RNApol & DNA separ#43; NTP → RNA elongation
完成 NMP之间的磷酸脂键的连接
Editing
e.g. C-mycP2
Null
非激活转录
MELa I site
Pausing at +371 P
激活转录
MELa I factor
mRNA processing
4.4 Transcription termination
4.4.1 Termination in prokaryotes
Terminator: A sequence of DNA, presented at
Model of rho-dependent termination
a) Rho binds to the transcript and pursues the polymerase.
b) When the hairpin forms in the transcript, the polymerase pauses, giving Rho a chance to catch up.