CHAPTER 18 REGULATION OF GENE EXPRESSION18.1 Bacterial regulationI. Intro

A. Genes are controlled by an on/off “switch”1. If on, the genes can be transcribed2. If off, the genes cannot be transcribed

B. RNA polymerase has restricted access to DNAII. Operons: The basic concept

A. Operator1. The “switch” that controls DNA transcription2. A segment of DNA located within the promoter3. May control 1 gene or several related genes

B. Operon1. Operator + promoter + gene(s)

C. Operating the switch1. Repressor (“switch” protein)

a. Attached to operator= operon repressed (turned off)Not attached= operon not repressed (turned on)

b. Operator specific

c. Regulatory genes1. Genes located away from operon that code for “switch”

proteins2. Continuously making the proteins

III. Repressible & inducible operons: Two types of negative gene regulationA. Repressible operons

1. Operons are turned on by defaulta. No repressor attached to operatorb. RNA polymerase can attach & transcribec. To turn off a repressor protein attaches to operator

1. Blocks RNA polymerase attachment2. Reversible3. Allosteric protein

a. Active & inactive shapesb. Inactive state by default

1. Corepressor needed to activate

2. Examplea. E coli & AA tryptophan trp operon

1. Metabolic pathway

2. Gene regulation

a. trpR = regulatory gene1. Codes for inactive repressor

a. Corepressor (tryptophan) attaches to repressor making it active thus operator switch off

B. Inducible operons1. Operons are turned off by default

a. Repressor attached to operatorb. To turn on an inducer bonds to repressor inactivating it

1. Allosteric proteina. Active state default

2. Examplea. lac operon

IV. Cyclic AMP + CAP: A type of positive gene regulationA. Preferred energy source is glucose

1. Lactose if glucose is lackingB. Glucose present

1. Glycolysis proceeds as normal2. Bacteria provided with ATP

C. Glucose lacking/lactose present1. Bacteria need to change energy source

a. Need enzymes for lactose breakdown

2. cAMP [ ] increases with a lack of glucosea. Activates CAP

1. An activator2. Binds to promotor of lac operon

a. Increases affinity of RNA polymerase for promotorof lac operon1. Increases rate of transcription

D. CAP directly stimulates gene expression1. Positive regulation

V. Lac operonA. Under dual control

1. Negative control by lac repressora. Determines whether lac operon is transcribed at all

2. Positive control by CAPa. Determines the rate of transcription

18.2 Eukaryotic gene expression can regulate at any stage

I. Regulation of chromatin structureA. DNA is found in what state in non-dividing cells

1. Heterochromatin2. Euchromatin

B. Chemical modifications of histones & DNA1. Histone modification

a. Heterochromatin occurs due to the histonesbonding with neighboring nucleosomes1. Chromatin becomes tightly compacted

b. Histones Acetylation1. Acetyl groups (-COCH3) attach to histones2. Heterochromatin loosens into euchromatin3. DNA transcription can occur

2. DNA Methylationa. Certain DNA bases become methylatedb. Prevents transcriptionc. 1 potential cause for cell differentiationd. Can be passed on during cell division