SSR COLLEGE OF ARTS COMMERCE AND SCIENCE - PowerPoint Presentation

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SSR COLLEGE OF ARTS COMMERCE AND SCIENCE

DEPARTMENT OF MICROBIOLOGY

CLASS -TYBSC

SUBJECT- Genetics and Molecular Biology -I

Lecture-TRANSCRIPTION

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RNA polymerase

Core polymerase

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+2 +3 +4 -4 -3 -2 -1

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-70 to +30

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INITIATION

–11 to +2

isomerization

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ELONGATION

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Two proofreading functionspyrophosphorolytic editing-back-reaction, to catalyze the removal of an incorrectly inserted ribonucleotide,By reincorporationof Ppi

hydrolytic

editing- polymerase

backtracks by one or more nucleotidescleaves

the RNA product, removing the error-containing sequence

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Elongation Factors- Gre factors -

stimulate Hydrolytic editing Serve as elongation stimulating factors by helping to

overcome

“

arrest” at sequences that are difficult to transcribe

Nus

proteins-

promotes

, in still rather undefined ways, the

processes

of

elongation and termination

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Termination Terminators- sequences that trigger

the elongating polymerase to dissociateFrom the

DNA and release the RNA chain it has made

Rho-dependent-

requires the action of the Rho factor

rut sites

for Rho utilization , stretches of 40

nucleotide on RNA transcript

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Rho-independent/intrinsic terminators they need no other factors to work

Consist of two sequence

elementsinverted repeat (of 20 nucleotides

) stretch of about eight A:T base

stem-loop

structure/ “hairpin”

pairs

they

function in the RNA rather

than in

the DNA.

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TRANSCRIPTION IN EUKARYOTESpolymerases closely related to the RNA polymerases found in prokaryotesprocess of transcription itself is identical in the two

cases

Bacteria Eukaryotes only one RNA polymerase at

least three different ones (Pol I, II, and III)

only one additional initiation factor (sigma), several initiation factors, general transcription

factors

(GTFs

)

general transcription factors are not alone sufficient to bind

promoter sequences.

additional factors

are required,

including

DNA-binding regulatory proteins,

the

so-called

Mediator complex

,

and

often chromatin-modifying enzymes

.

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Eukaryotic core promoterminimal set of sequence elements

required for accurate transcription initiation

by the Pol II machinery40–60 nucleotides long

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Regulatory sequences-there are other sequence elements required for accurate and efficient transcription in

vivo

PromoterproximaL elements, Upstream activator

sequences (UASs)

enhancers, and a series of other elements called silencers,

activators

and repressors

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Preinitiation complex TBP (TATA-binding protein)

TFIID-TATA element-

TAFs, for TBP-associated

factors

Inr, DPE, and DCE,

TFIIA,

TFIIB,

TFIIF together with

polymerase,

TFIIE and TFIIH.

Formation of the

preinitiation complex

is

followed by promoter

melting

requires

hydrolysis of ATP

and is mediated by

TFIIH

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Promoter escapeATP hydrolysisphosphorylation of the polymerase

carboxy-terminal domain (

CTD)-Heptapeptide repeats –

Tyr-Ser-Pro-Thr-Ser-Pro-Ser.

27 in

the yeast

32 in

C

elegans,

45 in

Drosophila,

52

in humans.

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Elongationelongation factors Factors required for RNA processing

the kinase

P-TEFb

-phosphorylates the serine residue at

position 2 of the CTD repeats -

phosphorylates and thereby activates

another protein, called SPT5

,

TAT-SF1

, yet another elongation factor, is recruited by

P-TEFb .

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Post transcription Modifications Capping

addition of a modified

guanine base to the

5’ end of the RNA,

methylated guanine

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polyadenylation

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Transcription TerminationRNAase in yeast, Rat in humans, Xrn2

protein (

Rtt103) loads

torpedo

allosteric.

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RNA Splicing

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introns and exons borders

four classes of introns- group I and

s

elf-splicing group

II introns

Splicosomal Introns

t RNA , endonuclease ,ligase

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Splicing mechanism of group Iintrons

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Splicing mechanism of group IIintrons

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Spliceosomal intronsSpliceosome 5 Small nuclear RNAs (snRNAs), 100 and

300 nucleotides lon 150

proteins

snRNAs

- U1, U2, U4, U5, and U6

snRNPs “snurps

-

SnRNAs

+ Proteins

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ECB

A

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Regulation OF Transcription Regulatory Proteins positive regulators, or activators negative regulators, orrepressorsBasal level transcription – Promotor sequence

gene expression

constitutive- housekeeping

genes regulated - inducible, repressible.

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