Chapter 6

Chapter 6

Getting the Message Out:

Transcription of Genes to Produce Messenger RNA

Transcription : the first step of gene expression

Gene expression : the process by which information from a gene is used in the synthesis of a functional gene product

Gene product : the biochemical material, either RNA or protein, resulting from

expression of a gene

Gene of interest

Pull strands apart locally

RNA strand Is made

ss RNA made (mRNA)

Template strand

Transcription: The principle

RNA

: ribonucleic acid mRNA

: messenger RNA Transcription

: process by which information from DNA is converted into its RNA equivalent

Chemical differences b/t DNA & RNA

Ribose Deoxyribose

1. Sugar

Thymine Uracil

2. Base

3. Number of strand

Double strand vs Single strand

Template strand( 주형가닥 ): strand of DNA that is read during transcription

Coding strand( 암호화가닥 ): non-template strand

“Sequence of the new RNA molecule is identical to

the sequence of the coding strand of DNA”

Binding of RNA polymerase

Direction of RNA polymerase

Template strand of DNA

Non-template strand of DNA (i.e. coding strand) RNA polymerase

: enzyme that synthesizes RNA using a DNA template

Upstream region Down stream region

Promoter Gene

first base of the gene

upstream & downstream

- 10 region - 35 region

Recognized by RNA polymerase promoter

: sequence in front of a gene which RNA polymerase binds to

Recognition of -10 & -35 regions by sigma

Core enzyme

sigma

Sigma subunit

: subunit of bacterial RNA polymerase that recognizes and binds to the

promoter sequence Core enzyme

: RNA polymerase of bacteria without its sigma subunit

Consensus sequence

: ideal sequence to which real life sequences approximate

Consensus sequences Holoenzyme

( 거대효소 )

TATA box

그림 6-7

Synthesizing the message - elongation

“Once sigma has found a promoter and the RNA polymerase has successfully bound to it, the sigma subunit drops off. The remaining part of bacterial RNA polymerase known as the core enzyme, then makes the mRNA”

“U instead of T”

Roles of RNA polymerase 1. Synthesis of RNA

2. Unwinding & winding DNA [no proofreading ability, cf) DNApol I

그림 6-8

How does RNA polymerase know where to stop?

Recognition sequence 1. inverted repeats 2. a run of As

Inverted repeats make a stem and a loop

“Hairpin” structure

Terminator

그림 6-10

Termination of transcription

Hairpin structure

House keeping gene

-genes that are in constant use to maintain basic cell functions -expressed “constitutively”

-ex) beat-actin, GAPDH, ubiquitin

Others

-genes which are only needed under certain condition

-slightly different recognition sequences in the -10 and -35 regions -not recognized by the sigma subunit unless other accessory protein is there to help.

-these proteins are “gene activator proteins”

-in eukaryotes, genes are often controlled by transcription factors

그림 6-11

Activator proteins:

그림 6-12

An example of activator protein

active form

Positive regulation

그림 6-13

Repressor proteins:

(operator sequence

: repressor binding site)

Inactive form

“The repressor blocks the binding of RNA polymerase, or prevent it from moving forward”

Negative regulation In the absence of lactose

In the presence of lactose

그림 6-14

Most regulator proteins bind small molecules & change shape

Signal molecule Signal binding site

DNA binding site

Signal molecules => Allosteric proteins

그림 6-15

Most regulator proteins have two or four subunits

Non-binding form DNA binding form

DNA recognition site

그림 6-16

Inverted repeat binding

그림 6-17

Global regulation

: regulation of a large group of genes in response to the same environmental stimulus Global regulator protein

: protein that controls expression of many genes in response to the same signal

총체적 조절 : 유전자 그룹의 발현 조절

그림 6-18

Crp protein

: a global activator for the control of the use of alternative sugars to glucose : signal molecule - cAMP

: binds to cyclic AMP

: cyclic adenosine monophosphate receptor protein : a global signal that the cell has run out of glucose

: cAMP: regulatory nucleotide (due to the structural similarity), act as a signaling molecule

cyclic AMP

그림 6-19

lac Operon

Naming genes : 3 letters

: italic

: small letter

Operon

: a characteristic of bacterial gene expression (not found in higher organisms) : a cluster of genes transcribed together to give a single molecule of mRNA : same promoter (same starting point)

: one operon has one or more genes

β-galatosidase, lactose permease, galactoside-O-acetyl transferase

그림 6-21

ON

OFF

OFF

OFF

Antisense RNA

: RNA that is complementary in sequence to messenger RNA and therefore base pairs with it

: negatively regulates transcription : prevent mRNA from binding to ribosome and therefore block protein synthesis

: not made by transcribing the coding strand of the same gene as the mRNA. There is “anti-gene” for antisense RNA

: under investigation to treat chronic diseases such as cancer

luciferase

Synthesis of auto-inducer (signal molecule)

Quorum sensing

: form of regulation in which a gene is expressed in response to

population density

Auto-inducer

: signal molecule which can freely exit and enter cells and is involved in quorum sensing

Luciferase

: enzyme involved in a light emission reaction

:ex. Vibrio fischeri, Firefly

: widely used in modern molecular biology (reporter gene, chapter 16)

luciferin

luminescence

그림 6-24

• 오페론 (operon) 이란 유전체 DNA 상에서 기능적으로 연관된 유전자들이 하나의 전사단위 로 묶여 서로 이웃하여 존재하는 구조의 기능적 단위를 의미한다 . 대개 프로모터

(promoter), 작동유전자 (operator), 구조유전자 (structural gene) 의 세 부분이 DNA 상에 일렬로 연결되어 있으며 이들을 일괄적으로 조절하는 조절유전자(regulatory gene) 가 따로 존재한다 . 대개는 세포 내에서 같은 생화학적 반응에 관여하는 여러 효소들이 오페론 구조에 의해 하나의 전사체 (transcript) 로부터 만들어지므로 세포의 필요에 따라 화학양론 적으로 같은 양의 효소를 만들어낼 수 있다 . 1961 년 프랑수아 자코브 (Francois Jacob) 와 자크 모노(Jacques Monod) 가 대장균에서 락토스 ( 젖당 ) 오페론 (lac operon) 이 존재함 을 처음 밝혀내었고 , 이 공로로 1965 년 노벨생리의학상을 수상하였다 . 오페론의 예로는 대표적으로 락토스 오페론과 트립토판 오페론 (trp operon) 등을 들 수 있으나 , 원핵세포에 서는 대부분의 유전자들이 오페론으로 구성되고 있으며 , E. coli K12 와 Bacillus subtilis(

고초균 ) 의 유전체에는 각각 약 600 개 정도의 오페론이 존재한다는 것이 알려져 있다 ( 그림 1). 오페론 구조는 원핵생물에서만 존재하는 것으로 알려져 왔으나 최근에 선충

(Caenorhabditis elegans) 과 초파리 (Drosophila melanogaster) 등의 고등 진핵생물 의 일부 유전자와 바이러스에서도 발견됨으로써 원핵생물만의 특징이라 말할 수 없게 되었다 .

• [ 네이버 지식백과 ] 오페론 [operon] ( 미생물학백과 )