Chapter 18

Concept 18.1: Environment influences bacteria transcription

Natural selection favors Conservation of energy and resources in bacteria.
Cell regulates activity of enzymes by feedback inhibition or by gene regulation.
- Feedback inhibition: end product of metabolic pathway shuts down further synthesis.
- Gene regulation: enzyme production is controlled/regulated.
By default, the trp operon is on and tryptophan ammino acid is made.
When tryptophan is preset, it binds to trp repressor protein which stops production of AA (since its not needed) by turning on repressor.
Repressor is in active state only in presence of its corepressor tryptophan.
Thus trp operon is repressed or stopped from producing.

A repressible operon is one that is usually on but binding of a repressor to a operator turns it off.
- Trp is an repressible operon.
Inducible operon is one that is usually off until an inducer activates the repressor which turns on operon.
- Lac repressor/operon is an inducible operon that is activated by the presence of lactose, allowing transcription of genes required for lactose metabolism.
Inducible Operons usually only function in catabolic pathways - always activated by a chemical signal.
Repressible enzymes usually only function in anabolic pathways, being repressed by high concentration of end product.
Regulation of trp and lac are negative gene regulation , where the binding of a repressor protein to the operator sequence inhibits transcription.

Some operons are also subject to positive control through a stimulatory protein, such as cyclic AMP receptor protein (CRP).
When glucose is scarce, CRP is activated by binding w/ cyclic AMP (cAMP).
- Low-food → Low Glucose → CRP + cAMP → operon activation → production of glucose
Activated CRP attaches to the promoter of lac operon and increases the affinity of RNA polymerase.
- Jumpstarts transcription… more glucose.
- trp and lac are a focus, CRP is priority 2.

All organisms must regulate which genes are expressed at any given time.
Genes are turned on and off in response to signals… internal & external cues.
Multicellular organisms, regulation of gene expression is essential for cell specialization… cells → tissues → organs → organ systems.

Almost all the cells in an organism contain an identical genome.
Differences between cell types result from differential gene expression.
Abnormalities in gene expression can lead to diseases like caner.
Gene expression is regulated at MANY stages.. but happens most often @ transcription.