CONTROL MECHANISMS Sections 5.5 Page 255

Outline Gene regulation Operons lac operon trp operon Recap

Gene regulation Housekeeping genes Always needed constantly being transcribed/translated Ex. Single stranded binding proteins Most genes are not housekeeping genes

Gene regulation involves turning certain genes on or off depends on whether their products are required Regulators are the proteins that switch genes on or off. The 2 types of regulators are: Activators (turn genes on) Repressors (turn genes off)

Recall… Protein synthesis occurs in two distinct stages: Transcription, in the _________ Product: ____ Translation, in the ______________ Product: __________

How do we control gene expression 4 Levels of control Transcriptional Posttranscriptional Translational Posttranslational

Levels (cont’d) Transcriptional gene regulation Regulation of which genes are transcribed Can involve control of the rate at which transcription occurs Posttranscriptional gene regulation Modifications made to primary transcript in the nucleus

Take a break… Can you make an educated guess about: Translational control?? Post-translational??

Levels (cont’d) Translational gene regulation Control of how often and how rapidly mRNA transcripts are translated into proteins Posttranslational gene regulation Modifications made to the polypeptide chain

Transcriptional regulation Two classic examples: lac operon – negative regulation trp operon – positive regulation

only in prokaryotic cells What is an operon? a cluster of genes all genes are under the control of one set of regulatory sequences promoter and operator sequences regulation is dependent on the presence or absence of effector molecules

Operator sequence Sequence to which regulator protein binds Depending on the regulator, binding to the operator causes either: activation of transcription repression of transcription

Effector Acts on the regulator protein Can be an inducer stimulates transcription Can be a co-repressor inhibits transcription

THE LAC OPERON

Lac operon lactose = glucose + galactose disaccharide found in milk E. coli: ß-galactosidase catalyses the cleavage of the bond in lactose Not economical to produce ß-galactosidase when lactose is not present

Lac operon (cont’d) Negative control mechanism prevents production of ß-galactosidase if lactose is absent

Structure of lac operon Cluster of genes: lacZ, lacy, lacA Each codes a different part of the enzyme ALL are under the control of one promoter

LacI protein – repressor protein LacI protein – repressor protein. Blocks transcription of the lac operon by binding to the operator. Physical blockage: Covers part of the lac promoter

Mechanism of regulation http://www.sumanasinc.com/webcontent/ animations/content/lacoperon.html

Mechanism of regulation

Mechanism of regulation lactose is absent lactose is present LacI binds to lac promoter. No transcription. Lactose binds to LacI protein. LacI changes conformation → unable to bind lac operator. RNA polymerase can access lac promoter; transcription occurs.

What is the effector molecule? _______________ It acts as an inducer – it induces transcription by deactivating the repressor protein.

Negative control Default: Transcription is off, due to binding of a repressor protein. Presence of an effector (inducer) removes the repressor from the operator region. Transcription can now occur.

TRP OPERON

The trp operon Tryptophan – Amino acid Operon: cluster of five genes encoding enzymes required for trp synthesis Transcription of operon is repressed when concentrations of trp in the cell are high

Positive control mechanism prevents production of tryptophan, trp operon (cont’d) Positive control mechanism prevents production of tryptophan, if tryptophan levels are high http://highered.mcgraw- hill.com/olc/dl/120080/bio26.swf

Mechanism of regulation

Mechanism of regulation trp absent trp present trp operon is transcribed (default) trp itself will bind to the repressor protein. conformational shape occurs allows repressor to bind to the trp operator → shuts off transcription

What is the effector molecule? _______________ It acts as a co-repressor. It binds to a repressor to activate it in order to repress transcription.

Positive control Default: Transcription is on Presence of an effector (co-repressor) causes binding of the repressor to the operator. Transcription does not occur.

trp vs. lac operons Default state Effector Positive control Transcription is on Allows repressor to bind Negative control Transcription is off Removes repressor Positive vs. Negative Control: Is the repressor added or taken away?

Recap: Vocabulary Basic terms: Gene regulation: Describes any process that alters the rate of gene expression. Operon: A cluster of closely-related genes. They are all controlled by one set of regulatory sequences.

Types of molecules involved: Regulators: Molecules that carry out translational gene regulation. Classified as either activators, or repressors. Example: LacI repressor; trp repressor Effectors: Substances to which regulator proteins respond. Inducers or Co-repressors Example: Lactose, Tryptophan

Regulatory DNA sequences: Promoter: DNA sequence to which RNA polymerase binds to begin transcription. Operator: DNA sequence to which a transcription factor binds in order to alter transcription. Close to the promoter.

Homework Pg. 258 #1-6