15 BIO 326R Operons

15 BIO 326R Operons - BIO 326R Operons Terms Operon a...

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BIO 326R Operons
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Terms Operon : a cluster of bacterial genes along with an adjacent promoter that controls the transcription of those genes. Operator : stretch of DNA, partially overlaps promoter and transcription start site
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How does a cell know what genes to  transcribe and when? 4 - 5,000 genes in E. coli, spread out over 4.6 million base pairs. Protein synthesis is energy intensive, need to keep number of genes transcribed to a minimum. Promoters can direct RNA polymerase to the correct place to start transcription, but how can promoters be turned on and off? Sigma factors are important for some of this Note most promoters are “leaky.” There will be at least some low level of transcription at all times, even when the regulatory machinery has turned transcription off. E. coli grown in a culture with glucose as a carbon/energy source and then switched to lactose: Cell division slows or stops briefly (lag phase) Cells do not initially metabolize lactose Soon, enzymes for metabolizing lactose are made in great quantity and cell division resumes 3 enzymes: Beta galactosidase (LacZ)– cleave lactose into galactose and glucose, also catalyzes a side reaction of lactose to allolactose. Lactose Permease (LacY) – transports lactose across the cell membrane Transacetylase (LacA) – function unknown
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Lac  operon map Operator Promoter LacI CAP LacZ LacY LacA Promoter Two promoters in this map, with 4 Protein coding regions Polycistronic mRNA under control of Lac Promoter encodes 3 genes: LacZ - beta galactosidase – cleaves lactose into glucose and galactose LacY - lactose permease – carrier protein to help lactose cross cell membrane LacA - Thiogalactoside transacetylase - unknown function LacI – Lac inhibitor, under control of leftward promoter Always on, but at a very low rate Promoter and operator sequences overlap slightly terminator
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promoter, but stops transcription Operator Promoter Lac I CAP LacZ LacY LacA Promoter Lac I RNA Pol Lac I
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15 BIO 326R Operons - BIO 326R Operons Terms Operon a...

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