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L51-07 Bacterial gene expression HANDOUT-6

L51-07 Bacterial gene expression HANDOUT-6 - Lecture 51...

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1 Lecture 51 Gene Expression in Prokaryotes --- Viruses Promoter binding provides the ultimate control of transcription 1. Certain DNA sequence in the promoter has stronger binding affinity for RNA polymerase 2. Many promoters are regulated by proteins - Repressors inhibit transcription initiation - Activators increase transcription initiation -50 Region AT-Rich TTGACA TATAAT UP-Element -35 Region 17-19 5-9 +1 Pribnow box -10 Region Repressor Repressors usually block transcription initiation by - interfering with RNA polymerase binding - preventing formation of open complex RNA polymerase Repressor mRNA X 3’ 3’ 5’ 5’ Recruitment : one surface of the activator binds to the site on DNA near the promoter, while with another surface the activator interacts with the RNA polymerase, bringing it to the promoter. Results : the closed complex turned into an open complex, facilitating transcription Activated level of transcription In eukaryotes genes for related functions are often located on different chromosomes and have different promoters Gene Organization in Eukaryotes Gene Organization in Prokaryotes - Operon: a contiguous array of related genes - One promoter controls transcription of multiple genes in the operon - One mRNA carries message for multiple enzymes for related functions - Consequently genes encoding enzymes in a common pathway can all be transcribed simultaneously
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2 1. Repressor inactive by default 2. Operon stays on, thus enzymes are made 3. When end product concentration is high, it binds to repressor to activate it 4. Repressor blocks transcription of enzymes for the production of end product Transcription Control: negative mode 1 mRNA enzyme X enzyme Y enzyme Z Precursor molecule P A B
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