1 31 Gene regulation in bacteria Lecture Outline 11/18/05 • Finish up from last time: • Transposable elements (“jumping genes”) • Gene Regulation in Bacteria – Transcriptional control – Cells adjust to their environment by turning genes on and off • The operon concept – Repressors, Inducers, Operators, Promoters • Repressible operons (e.g. trp) • Inducible operons (e.g. lac) Transposable elements • Normal and ubiquitous – Prokaryotes- • Genes transpose to/from cell’s chromosome, plasmid, or a phage chromosome. – Eukaryotes- • Genes transpose to/from same or a different chromosome. • Cause genetic changes – Chromosome breaks – Duplications – Knock-out genes I’ll talk about 2 kinds: • Insertion sequences • Ac/Ds elements in corn • A third major class: Retrotransposons – Uses RNA intermediate and reverse transcriptase – Most Important class in mammalian genomes
7
Embed
31 Gene regulation in bacteria Lecture Outline …dstratto/bcor011_handouts/31_operons.pdf6 •The lac operon: regulated synthesis ofinducible enzymes Figure 18.22a DNA mRNA Protein
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
31 Gene regulation in bacteria Lecture Outline 11/18/05• Finish up from last time:
• Transposable elements (“jumping genes”)
• Gene Regulation in Bacteria– Transcriptional control– Cells adjust to their environment by turning genes
Tryptophan present -> repressor active -> operon “off”.Figure 18.21b
Active repressor canbind to operator andblock transcription
Tryptophan changes the shape ofthe repressor protein so it can bindDNA
6
• The lac operon: regulated synthesis ofinducible enzymes
Figure 18.22a
DNA
mRNA
ProteinActiverepressor
RNApolymerase
NoRNAmade
lacZlacl
Regulatorygene
Operator
Promoter
Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.
(a)
5′
3′
mRNA 5'
DNA
mRNA
Protein
Allolactose(inducer)
Inactiverepressor
lacl lacz lacY lacA
RNApolymerase
Permease Transacetylaseβ-Galactosidase
5′
3′
(b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.
mRNA 5′
lac operon
Figure 18.22b
Positive Gene Regulation
• Both the trp and lac operons involve negativecontrol of genes– because the operons are switched off by the
active form of the repressor protein
• Some operons are also subject to positivecontrol– Via a stimulatory activator protein, such as
catabolite activator protein (CAP)
7
Promoter
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized.If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces large amounts of mRNA for the lactose pathway.
(a)
CAP-binding site OperatorRNApolymerasecan bindand transcribe
InactiveCAP
ActiveCAPcAMP
DNA
Inactive lacrepressor
lacl lacZ
Figure 18.23a
– In E. coli, when glucose is always the preferredfood source
– When glucose is scarce, the lac operon isactivated by the binding of the catabolite activatorprotein (CAP)
Positive Gene Regulation- CAP • When glucose is abundant,– CAP detaches from the lac operon, which
prevents RNA polymerase from binding tothe promoter