Transcription in eukaryotes

transcription process in eukaryotes their factors RNA polymerase enzyme an intiation elongation termination process

1. TRANSCRIPTION IN EUKARYOTES Mrs. Praveen garg VITS College, Satna

2. INTRODCTION Eukaryotic transcription  is the process that eukaryotic cells use to copy genetic information stored in DNA into complementary   RNA. Unlike prokaryotic RNA polymerase, In eukaryotes, there are three RNA polymerase. A eukaryotic cell has a nucleus that separates the processes of transcription and translation. Eukaryotic transcription occurs within the nucleus where DNA is packaged into  nucleosomes  and higher order chromatin structures.

3. Name Location Product RNA Polymerase I  (Pol I, Pol A) nucleolus larger ribosomal RNA ( rRNA) ( 28S ,  18S ,  5.8S ) RNA Polymerase II  ( Pol II, Pol B) Nucleus messenger RNA ( mRNA ), most small nuclear RNAs ( snRNAs ), small interfering RNA ( siRNAs ) and microRNA ( miRNA ). RNA Polymerase III  (Pol III, Pol C) nucleus (and possibly the nucleolus- nucleoplasm  interface) transfer RNA ( tRNA ), other small RNAs (including the small  5S ribosomal RNA (5s rRNA ) , snRNA . TYPES OF RNA POLYMERASE

4. EUKARYOTIC PROMOTERS The promoters recognized by RNA Polymerase II are larger and diverse. GC Box: These are structural gene, expressed in all tissues, also called as house keeping gene having sequence GGGCGG located upstream to transcription start site. TATA Box: It is structural gene present 25 to 30bp upstream to the start site, has consensus sequence TATAAAA. CAAT Box: It has GGCCAATCT sequence. It is located 70 to 80bp upstream to start site. Octamer Box: It is present in the RNA polymerase II promoter and has consensus sequence ATTTGCAT. It influence the efficiency of promoter to initiate transcription.

5. Promoter for RNA Polymerase II

6. Eukaryotic genes also contain regulatory sequences with the core promoter. These  regulatory elements bind transcriptional activators or repressors to increase or decrease transcription from the core promoter. These regulatory elements include enhancers,  silencers. These regulatory sequences can be spread over a large genome, sometimes located hundreds of kilobases from the core promoters . These factors have DNA-binding domains that bind specific sequence elements of the core promoter and help recruit RNA polymerase to the transcriptional start site. The position of the enhancers can be either upstream or downstream. Regulatory sequence

7. Transcription factors are a group of proteins involved in transcription initiation and regulation with RNA polymerase. Transcription factor are responsible for recognizing the promoter for all RNA polymerase. Initiation of transcription involve many protein-protein interaction among transcription factor bound at the promoter or at the enhancer with RNA polymerase. There are four type of transcription factor: Basal factors: required for synthesis of all mRNA. Activators: bind directly to DNA at promoter or the enhancer. Coactivators : bind to both activators and basal apparatus. Regulators: acts on chromatin structure. TRANSCRIPTION FACTOR

8. Basal transcription factor These factors required for initiation, it recognize initiation site and delivers RNA polymerase II to form a initiation complex. There are various transcription factor: TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH TFIIA: Stabilizes TBP and TAF binding TFIIB: Stabilizes TBP binding, influence start site selection TFIID: Recognize TATA box, recruit TFIIA and TFIIB, has positive and negative regulatory functions. TFIIE: It is heterotetramer consist of 2 α&2β unit, recruit TFIIH and stimulate its helicase activity; enhancer promoter melting. TFIIF: Facilitates promoter targetting stimulates elongation, recruit RNA polymerase II. TFIIH: Contain an ATP- dependent helicase that function in promoter melting and clearence .

9. Transcription factor and RNA Polymerase- II bind with promoter

10. Activators Activators are the transcription factors that bind to specific DNA sequences (GC box, CAAT box) upstream of the initiation site to stimulate or repress. They also bind to site in the promoter or in enhancer. They increase the efficiency of transcription and are required for a promoter to function properly. Some activators act constitutively, whereas others have a regulatory role and are synthesized or activated at specific time or in specific tissues. These factor are responsible for control of transcription pattern .

11. Activator have two independent domain DNA binding domain Activating domain

12. The role of DNA binding domain is to bring the transcription activation domain into the vicinity of promoter. Transcription activating domain work by making protein- protein contacts with transcription factor.

13. Co-activators Members are another group of factors necessary for efficiency of transcription do not themselves bind DNA. Co-activators provide a connection between activators and the basal apparatus. They work by protein-protein interaction and forming a bridges between activators and basal apparatus.

14. In the process of initiation, transcription factors firstly bind to the promoter region and then help recruit the appropriate RNA polymerase. The completed assembly of transcription factors and RNA polymerase bind to the promoter, forming a transcription pre-initiation complex (PIC). The TATA box, as a core promoter element, is the binding site for a transcription factor known as TATA-binding protein (TBP), a subunit of another transcription factor: TFIID. After TFIID binds to the TATA box via the TBP, five more transcription factors and RNA polymerase combine around the TATA box in a series of stages to form a pre-initiation complex. INITIATION OF TRANSCRIPTION

15. Formation of Pre-Initiation Complex

16. TFIIH is involved in separating opposing strands of double-stranded DNA to provide the RNA Polymerase access to a single-stranded DNA template.  Other proteins known as activators and repressors, along with coactivators or corepressors , are responsible for modulating transcription rate. Activator proteins increase the transcription rate, and repressor proteins decrease the transcription rate.

17. RNA Polymerase II is a complex of 12 protein subunits. Specific subunits allow RNA Polymerase II to act as its own helicase , sliding clamp, single-stranded DNA binding protein, as well as carry out other functions.  Transcription elongation occurs in a bubble of unwound DNA, where the RNA Polymerase uses one strand of DNA as a template to catalyze the synthesis of a new mRNA strand in the 5′ to 3′ direction. Early in the elongation process, the 5’ end of the pre mRNA are modified by the addition of 7- methyl guanosine (7-MG) caps. The 7- MG caps are added when the growing RNA chain are only 30 nucleotide long. It helps to protect mRNA chain from degradation by nucleases. This process continues until transcription termination occurs. ELONGATION OF TRANSCRIPTION

18. 7-MG

19. The 3’ ends of RNA transcripts synthesized by RNA polymerase II are produced by endonucleolytic activity cleavage of the primary transcript. Transcription termination occur in downstream site from the start point that will become 3’ end of the mature transcript. Towards 3’ end of the primary transcript usually contain nucleotides sequence AAUAAA, consensus sequences, which help in cleavage process of pre mRNA. After cleavage the enzyme poly A polymerase adds poly A tail, to the 3’ end of transcripts. The addition of poly A tail to mRNA is called polyadenylation . This process play a important role in their transport from the nucleus to the cytoplasm. TERMINA TION OF TRANSCRIPTION

20. THANK YOU