RNA Polymerase Slides
5 likes12,306 views
The document details the role and function of RNA polymerases in transcription, describing how RNA polymerase binds to promoter sequences to synthesize RNA from DNA. It differentiates between prokaryotic and eukaryotic RNA polymerases, noting that prokaryotes use a single type while eukaryotes have three distinct classes, each with unique properties and functions. Additionally, it outlines the transcription process, including initiation, elongation, and termination phases, highlighting the roles of various transcription factors.
RNA Polymerase Slides
- 2. Key points • Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule. • RNA polymerase is the main transcription enzyme. • Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins). • RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule. • Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished. Learning Garden
- 3. Introduction • Ribonucleic acid polymerase is an enzyme which is responsible for the production of all kinds of RNAs such as messenger RNA, ribosomal RNA and transfer RNA. • It is the largest enzyme of 490,000 MW and consists of six subunits made up of polypeptide chains: 2 alpha, beta, beta dash, omega and sigma subunits. • It catalyzes the process of transcription. It copies the DNA sequence into RNA sequence. Learning Garden
- 4. Introduction • The complete RNA polymerase enzyme is termed holoenzyme and can be represented as α 2ββ´ωσ in which attachment of sigma (σ) subunit (or factor) is not very firm, but resultant core enzyme (α2ββ´) does not lose its catalytic activity of transcription. • RNA polymerase found in prokaryotes, eukaryotes, archea and in some viruses is almost similar, this shows that they had evolved from a common ancestor. Learning Garden
- 6. Functions of RNA polymerase • Involved in the production of molecules which have a wide range of roles • Involved in post-transcriptional modification of RNAs to make them functional and facilitates their export from the nucleus towards their ultimate site of action. • The beta and beta dash subunits of RNA polymerase form the catalytic center of RNA-P and helps in unwinding of DNA molecule for transcription. • The sigma (σ) factor helps in the recognition of start signals on DNA molecule and directs RNA polymerase in selecting the initiation sites (promoter). Learning Garden
- 7. Prokaryotic RNA polymerase • A single RNA polymerase enzyme is involved in the production of all kinds of RNAs in E.coli and other prokaryotes. • It is the largest enzyme of 490,000 MW in prokaryotes. • The RNA polymerase of prokaryotes is made up of six subunits which include a sigma factor which is dissociated from enzyme complex after initiation of transcription. • prokaryotes use the same RNA polymerase as a catalyst in polymerization of coding as well as non- coding RNA Learning Garden
- 8. Prokaryotic RNA polymerase Learning Garden
- 9. Eukaryotic RNA Polymerase • In eukaryotes, there are three major classes of RNA polymerases which are designated as I, II and III and are found in the nucleus. • The three polymerases have different properties and can be distinguished by the ions required for their activity, the optimal ion strength and their sensitivity to inhibition by various antibiotics (e.g., α -amanitin). • Each of these enzymes is a large protein ( ~ 500,00 daltons), with two large and several ( 8 to 10) smaller subunits. Learning Garden
- 10. Table 1. Locations, Products, and Sensitivities of the Three Eukaryotic RNA Polymerases RNA Polymerase Cellular Compartment Product of Transcription α-Amanitin Sensitivity I Nucleolus All rRNAs except 5S rRNA Insensitive II N21ucleus All protein-coding nuclear pre- mRNAs Extremely sensitive III Nucleus 5S rRNA, tRNAs, and small nuclear RNAs Moderately sensitive Types of Eukaryotic RNA Polymerases Learning Garden
- 11. Types of Eukaryotic RNA Polymerases Learning Garden
- 12. RNA polymerase I • It is located in the nucleolus, it is a specialized nuclear substructure in which transcription and assembling of ribosomal RNA into ribosomes takes place. • The ribosomal RNA molecules have a cellular role but are not translated into protein so they are considered structural RNAs. • The rRNAs are components of the ribosome and are essential to the process of translation. • RNA polymerase I synthesizes all of the rRNAs except for the 5S rRNA molecule. Learning Garden
- 13. RNA polymerase II • It is found in nucleus and it synthesizes all protein-coding nuclear pre- mRNAs and is extremely sensitive. • Eukaryotic pre-mRNAs undergo extensive processing after transcription but before translation (as shown in the next slide). • RNA polymerase II is involved in the transcription of eukaryotic genes. Learning Garden
- 14. Eukaryotic mRNA contains introns that must be spliced out. A 5′ cap and 3′ poly-A tail are also added. Learning Garden
- 15. RNA polymerase III • It is also located in the nucleus of the cell. It transcribes small nuclear pre- RNAs and variety of structural RNAs which includes the 5 S pre-rRNA and pre-tRNA . • The function of small nuclear RNA is in splicing and the functions of rRNA and tRNA is to synthesize prorteins and translate the mRNA to form the polypeptide chain, respectively. Learning Garden
- 16. Comparison between DNA and RNA Polymerase • The roles of DNA and RNA polymerases is almost similar i.e. to catalyze the polymerization reactions taking place in nucleus, however there are two differences between them in their activity. • There is no need of primer to start the polymerization in case of RNA polymerase enzyme. • On the other hand DNA polymerase requires primer to begin transcription. • RNA polymerase enzyme is capable of reaction from the middle part of DNA strand. • It also reads the stop signals through which it dissociate itself from the template. Learning Garden
- 17. RNA polymerase comparison in prokaryotes and eukaryotes • Transcriptions in Eukaryotes and Prokaryotes have some similarities as well as some differences. The mechanisms of transcription are diverged in both. There are some characteristics of transcription which are common in both eukaryotes and prokaryotes which are as follows: • From E. coli to human the core is conserved • Active site is also conserved and magnesium ions are utilized by both polymerases to facilitate the process of transcription • Absorptive transcripts are found in both • Both binds to promoters and have elements to speed up transcription. Learning Garden
- 18. RNA polymerase comparison in prokaryotes and eukaryotes • There are many differences between prokaryotic and eukaryotic polymerases which are as follows: • Eukaryotes have three RNA polymerases which are RNA polymerase I, RNA polymerase II and RNA polymerase III while prokaryotes have only one RNA polymerase. • The molecular weight difference in RNA polymerase of prokaryotes and eukaryotes is 100 kDa. • There are six subunits of RNA polymerase in prokaryotes (2 alphas, beta, beta dash, an omega and a sigma) while RNA polymerase have twelve subunits in eukaryotes (Rpb1- Rpb-12). • The bridge helix in prokaryotes is in bended form while it is straight in eukaryotes. • ` 1 Learning Garden
- 19. Comparison between DNA and RNA Polymerase • The roles of DNA and RNA polymerases is almost similar i.e. to catalyze the polymerization reactions taking place in nucleus, however there are two differences between them in their activity. • There is no need of primer to start the polymerization in case of RNA polymerase enzyme. • On the other hand DNA polymerase requires primer to begin transcription. • RNA polymerase enzyme is capable of reaction from the middle part of DNA strand. • It also reads the stop signals through which it dissociate itself from the template. Learning Garden
- 22. Process of Transcription • The binding of RNA polymerase to DNA starts the transcription. The specific part of DNA at which transcription starts is called promoter region. • This binding takes place in the presence of other proteins such as sigma factor which works in prokaryotes and in eukaryotes various other factors helps in binding. • A set of protein is necessary for all transcriptional activity of eukaryotes. This set is called general transcription factors it includes transcription initiation factor II A, factor II B, factor II D, factor II E, factor II F and factor II H. Learning Garden
- 23. Process of Transcription • These are supplemented by specific signaling molecules that modulate gene expression through stretches of non-coding DNA located upstream. • Before a stretch of ten nucleotides is polymerized initiation is often aborted multiple times. • After this step, the polymerase loses most of the initiation factors and moves beyond the promoter. • After this step the unwinding of DNA takes place in which sort of bubble is formed where active transcription occurs. This process is also called melting. • The formed ‘bubble’ move along the DNA strand as the RNA polymer elongates. • When the process of transcription is complete, the process is stopped and RNA strand is processed. • The process of transcription is terminated by RNA polymerase III when there is a stretch of Thymine bases on the non-template strand of DNA. Learning Garden
- 24. Eukaryotic Transcription Mechanism Following are the steps of eukaryotic transcription mechanism: A. Initiation of Eukaryotic Transcription B. Elongation of RNA Chain in Eukaryotes C. Termination of Eukaryotic Transcription Learning Garden
- 25. Initiation of Eukaryotic Transcription • The eukaryotic transcripts are regulatory DNA sequences (such as promoters, enhancers and silencers) of the genes encoded by each of the three different RNA polymerases. • Different aspects of writing are also involved in the design of the writing space required for the initiation of writing. • Generally, each RNA polymerase is believed to have its own set of transcripts, however, TF II D or a portion thereof (e.g., TBP = TATA protein protein) is required for all three RNA polymerases . • Transcription factors (TFs) can be defined as proteins, which are required for the initiation of transcription, but not for the RNA polymerase component. T • hey assist in the synthesis of DNA polymerase RNA to form the so-called pre-activation complex or transcript. After the construction of this complex synthesis of text occurs. All known transcription factors may recognize DNA sequences, other elements or RNA polymerases. Learning Garden
- 26. Initiation of Eukaryotic Transcription • Transcriptosome formation by RNA pol II. The promoter sequence responsible for the positive expression of a common gene (also called a housekeeping gene) in all cells, is said to be a promoter of some people. Generic promotions cannot deliver controlled calls (e.g., tissue or a specific genetic attribute, called a primitive type). • Activation of normal promoter transcription by RNA polymerase II requires the action of different transcription factors (TFs) in the following order: (i) TF II D binds to the TATA box; (ii) step (i) to allow for the merging of TF IIA and TF IIB; (iii) TF II B forms the so- called DB complex and RNA polymerase II that link to the developmental site; (iv) RNA pol II interacts with the TF II F promoter to form a transcription factor; (v) the systematic addition of TF II E, TF II H and TF II J facilitates the initiation process. Learning Garden
- 27. Elongation of RNA Chain in Eukaryotes • There are certain transcription factor proteins, called elongation factor, that enhance the overall activity of RNA polymerase II and lead to an increase in elongation rate. • At least two proteins such as these are known: (1) TF II F accelerates RNA chain growth evenly in combination with RNA polymerase II. 2. The TF II S (also called S II) aids in the elevation of the RNA chain by removing the barrier along the entry pathway. • TF II S is known to act first by inducing hydrolytic cleavage at the end of the 3NA RNA, thus, assisting in the progression of RNA polymerase from any block to elongation. Learning Garden
- 28. Termination of Eukaryotic Transcription • In eukaryotes, the actual elimination of RNA polymerase II activity during transcription is possible through elimination sites similar to those found in prokaryotes. • However, the status of individual sites is unknown. Sites of such termination are believed to be located at a distance (sometimes up to one address away from the 3'end mRNA site). Learning Garden
- 29. Eukaryotic Transcription Mechanism Learning Garden