Vision Collage of Medicine Year 2, Level 3, Lecture 7: Transcription in Prokaryotes PDF

# Vision Collage of Medicine ## Year 2, Level 3, Lecture 7: Transcription in Prokaryotes ### by Dr. Ezat Mersal ## Objectives: - Transcription in Prokaryotes - Properties of Promoters in Prokaryotes and Eukaryotes - Comparison of Transcription in Prokaryotes and Eukaryotes ## Transcription in Prok...

# Vision Collage of Medicine ## Year 2, Level 3, Lecture 7: Transcription in Prokaryotes ### by Dr. Ezat Mersal ## Objectives: - Transcription in Prokaryotes - Properties of Promoters in Prokaryotes and Eukaryotes - Comparison of Transcription in Prokaryotes and Eukaryotes ## Transcription in Prokaryotes - There are Only single RNA polymerase - In *E.coli*, RNA polymerase is 465 kD complex, with 2 α, 1 β, 1 β', 1 ω - α binds DNA - β binds NTPs and interacts with ω - ω recognizes promoter sequences on DNA - β' subunits appear to be essential for assembly and for activation of enzyme by regulatory proteins - Polymerase is accurate - only about 1 error in 10,000 bases - Even this error rate is OK, since many transcripts are made from each gene. ## Stages of Transcription - Binding of RNA polymerase holoenzyme at promoter sites - Initiation of polymerization - Chain elongation - Chain termination ## Properties of Promoters - Promoters typically consist of 40 bp region on the 5'-side of the transcription start site - Two consensus sequence elements: - The "-35 region", with consensus TTGACA - sigma subunit appears to bind here - The Pribnow box near -10, with consensus TATAAT - this region is ideal for unwinding ## Properties of Promoters - Eukaryotic gene promoter sequences ## 1- Initiation of Polymerization - RNA polymerase has two binding sites for NTPs: - Initiation site prefers to binds ATP and GTP (most RNAs begin with a purine at 5'-end) - Elongation site binds the second incoming NTP - 3'-OH of first attacks alpha-P of second to form a new phosphodiester bond - When 6-10 unit oligonucleotide has been made, sigma subunit dissociates, completing "initiation" - Note rifamycin and rifampicin and their different modes of action ## 2- Chain Elongation - Core polymerase - no sigma - Elongation rate is 20-50 bases per second - slower in G/C-rich regions and faster elsewhere - Topoisomerases precede and follow polymerase to relieve supercoiling ## 3-Chain Termination - Two mechanisms - Rho - the termination factor protein - rho is an ATP-dependent helicase - it moves along RNA transcript, finds the "bubble", unwinds it and releases RNA chain - Specific sequences - termination sites in DNA - inverted repeat, rich in G:C, which forms a stem-loop in RNA transcript (Hair Pin) - 6-8 As in DNA coding for Us in transcript ## Termination strategies in bacteria - Rho-Dependent - Rho-Independent ## Rho - protein ## Hair Pin Sequence ## Comparison of Transcription in Prokaryotes and Eukaryotes | | Prokaryotes | Eukaryotes | |------------------|-----------------------------------|------------------------------------| | Transcription and translation | Occur simultaneously (coupled) | Occur separately | | Process | Simple (Polycistronic type of transcription) | More complicated (Monocistronic type of transcription) | | mRNA | No need of modification | Needs modifications | | Enzymes involved | RNA polymerase, β, β', σ | RNA polymerase; I, II, III (Three different types of RNA polymerase required for synthesis of all type of RNA) | | Termination | Well known | Less clear | | Site | Cytoplasm | Nucleus | | DNA | Circular and free | Linear, packed with histones | | RNA polymerase | Made up by 5 subunits | Made up by 10-15 subunits | ## Reference Books - Basic genetics: a human approach / BSCS. Dubuque, IA, Kendall/Hunt Pub. Co., c1999. 147 p. QH431.B305 1999 - Genes, ethnicity, and ageing. Edited by Lincoln H. Schmitt, Leonard Freedman, Rayma Pervan. Nedlands, Australia, Centre for Human Biology, University of Western Australia; Singapore, River Edge, NJ, World Scientific, c1995. 100 p.QH455.G45 1995 - Genetic polymorphisms and susceptibility to disease. Edited by M. S. Miller and M. Т. Cronin. New York, Taylor & Francis, 2000. 266 p.