Transcription and regulation phiên mã và điều hòa phiên mã bản dịch đính kèm

Cơ chế biên tậpTrước phiên mãGuide RNAs (gRNAs) định hướng biên tập:+ gRNAs là những phần nhỏ và bổ sung với mRNA được biên tập+ gRNA bắt cặp với RNA không được biên tập tạo ra vùng không khớp được các bộ máy biên tập nhận ra.Cải biến premRNA nhân thực: gắn mũPhần 7methylguanylate gắn vào nucleotide đầu tiên cua premRNA bởi liên kết 5’5’. (gắn guanin trước rồi mới methyl hóa guanin, thực tế khi vừa phiên mã được những nucleotide đầu tiên thì mRNA đã gắn mũ) Nhóm 2’hydroxyl của ribosome ở nucleotide thứ 2 ccũng có thể được methyl hóa. Mũ 5’ quan trọng cho vận chuyển mRNA ra tế bào chất, bảo vệ khỏi các enzym nuclease, khởi đầu dịch mã.

Transcription and Transcriptional Regulation

Hoang Thi My Hanh, Ph.D Cell Biology Department

The Central Dogma

FIGURE 11.1 The flow of genetic information according to the central dogma of molecular biology

Principles of Gene, 6th

Gene structure – Prokaryote vs Eukaryote

Principles of Gene, 6th

Eukaryote Gene Structure

Principles of Gene, 6th

Gene Expression in Prokaryote

Principles of Gene, 6th

Gene Expression in Eukaryote

Principles of Gene, 6th

Promoter selection is determined by the interaction of one or more transcriptional activator(s) with specific DNA sequences (recognition sites) near target genes Activators then recruit components of the transcription machinery to these genes through protein–protein interactions

Early steps in the transcription cycle

Activation of gene expression is induced by the sequential recruitment of large subunit protein co-activator complexes (shown in purple and pink) through binding to activators Activators also recruit ATP-dependent nucleosome-remodelling complexes, which move or displace histones at the promoter, facilitating the rapid recruitment and assembly of co-activators and the general transcription machinery

multi-Early steps in the transcription cycle

Regulation of transcription by promoter chromatin

disassembly and reassembly

Regulation of transcription by chromatin disassembly and

reassembly within transcription units

Together, co-activators and nucleosome remodellers facilitate the rapid recruitment of RNA polymerase II (Pol II) and the general transcription factors (GTFs) TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH to form the pre-initiation complex (PIC) on the core promoter9 These first three steps (a–c) constitute activator-dependent recruitment

Early steps in the transcription cycle

After PIC assembly, CDK7 in human TFIIH (Kin28 in yeast) phosphorylates the serine 5 (S5) position of the Pol II carboxy-terminal domain (CTD) At the same time, another subunit of TFIIH, the DNA helicase XPB (Rad25 in yeast), remodels the PIC, and 11–15 bases of DNA at the transcription start site (TSS) is unwound to introduce a single-stranded DNA template into the active site of Pol II83 Pol II then dissociates from some

of the GTFs and transitions into an early elongation stage of transcription83 This step is often referred to as promoter escape or clearance but is not sufficient for efficient passage of Pol II into the remainder of the gene

Early steps in the transcription cycle

| Following promoter clearance, Pol II transcribes 20 – 40 nucleotides into the gene and halts at the promoter-proximal pause site Efficient elongation by Pol II requires a second phosphorylation event at the S2 position of the Pol II CTD by CDK9,a subunit of human P-TEFb (Ctk1 in yeast)8, 104 Phosphorylation of the CTD creates binding sites for proteins that are important for mRNA processing and transcription through chromatin such as the histone H3 lysine 36 (H3K36) methylase SET2 (Ref 104) Nucleosome remodellers also facilitate passage of Pol II during the elongation phase of transcription The transcription cycle continues with elongation of the transcript by Pol II, followed by termination and re-initiation of a new round of transcription (not shown)

Early steps in the transcription cycle

Termination of RNA chain

RNA processing

Eukaryotic pre-mRNA Processing: Capping

a 7-methylguanylate residue

is attached to the first

nucleotide of the pre-mRNA

by a 5'-5' linkage The

2'-hydroxyl groups of the

ribose residues of the first 2

nucleotides may also be

methylated

The 5' cap is important for

transport of the mRNA to

the cytoplasm, protection

against nuclease

degradation, and initiation of

translation

The machinery splicing

Nat Rev Genet (2010)

ESS: Exon Splicing Silencer

ISS: Intron Splicing Silencer

ESE: Exon Splicing Enhancer

ISE: Intron Splicing Enhancer

Nat Rev Genet (2010)

Schematic representation of cis- and trans-splicing

Alternative trans-splicing in mammals

Editing Mechanism

• Post-transcriptional

• Guide RNAs (gRNAs) direct editing

– gRNAs are small and complementary to portions of the edited mRNA

– Base-pairing of gRNA with unedited RNA gives

mismatched regions, which are recognized by the editing machinery

– Machinery includes an Endonuclease, a Terminal UridylylTransferase (TUTase), and a RNA ligase

Guide RNAs Direct Editing

in Trypanosomes

Editing is from 3’ to 5’ along an unedited RNA

16.17,18

TUTase, or terminal uridylyl transferase, adds U(s) to the 3’ end created by cleavage of the pre-mRNA

from Fig 16.20

Editing Mechanism with the enzymes

RNA editing in brain tissue:

Adenosine to inosine

ADA

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

Bio 2050 – NQH, DDL, HTMN

DNA Methylation and Histone Modifications

Regulation of transcription by promoter chromatin

Disassembly and reassembly

2007: (17) 88-93

Regulation of transcription by promoter chromatin

Disassembly and reassembly