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