Regulation of Gene Expression Chromosomal Map begins at OriC; units of minutes. –Only structural genes for enzymes are shown here. –Their control regions.

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Presentation transcript:

Regulation of Gene Expression Chromosomal Map begins at OriC; units of minutes. –Only structural genes for enzymes are shown here. –Their control regions (promoter and operator) determine transcription. –The complete organizational unit is an operon. Transcriptional regulation: –Negative Control by Repressors Repression Induction –Positive Control by Activators –Attenuation (involves translation)

Transcriptional Regulation by Repression Regulatory protein (repressor) is encoded on a gene outside and away from the operon it regulates. Active repressor binds operator region; RNA Polymerase blocked = negative control. Repressor becomes active by a corepressor. Corepressor is often an endproduct of pathway enzymes encoded on the operon.

Transcriptional Regulation by Induction Active repressor binds operator region; RNA Polymerase blocked = negative control. Gene transcribed when inducer molecule is present; binds and inactivates repressor (release from operator). Inducers are typically substrate for a pathway enzyme encoded on the operon (e.g. allolactose for the lac operon)

Lactose Catabolism (lac) Operon Doesn’t work if glucose is available! Why?

Transcriptional Regulation by Catabolic Activator Protein (CAP) CAP = cyclic AMP receptor protein (CRP). Active CAP binds promotor and allows transcription to proceed = positive control. Activation of CAP requires build-up of cAMP to bind to CAP. cAMP builds-up in cells not producing enough ATP due to lack of glucose availability. The lac operon requires both lactose and cAMP.

lac Operon in Action (diauxic growth) PEP-PTS at high glucose uptake lowers adenyl cyclase activity; low cAMP; CAP inactive. Exhaustion of glucose increases cAMP, activating CAP; repressor is inactivated; lac operon transcribed! Separate cultures Together

Tryptophan (Trp) Operon (Trp synthesis (anabolic); regulated by repression and attenuation.)

Transcriptional Regulation by Attenuation In addition to a promotor and operator, the operon has a leader sequence with two pairs of self- complementing sequence sections (#1&2 and #3&4). The first pair is in what is called the leader peptide gene. The second pair (#3&4) is part of a Rho-independent terminator region upstream of any structural genes; called an attenuator. Trp high. Prevention of the first pair complementing will result in a hybrid complement of first and second pair (sections #2 and #3). Trp low.

Transcriptional Regulation by Attenuation Attenuation of transcription results when the attenuator hairpin can form. It forms when there is no translation of leader sequence mRNA & when there is ample trp-tRNA. Absence of trp-tRNA causes ribosome to stall, blocking section #1; hybrid forms. No attenuation hairpin; RNA polymerase proceeds to transcribe genes. 1) No Translation; No genes transcribed! 2) Trp & trp-tRNA available 3) Trp & trp-tRNA absent