Gene Regulation certain genes are transcribed all the time – constitutive genes synthesis of some proteins is regulated and are produced only when needed under special conditions

Gene Regulation in Prokaryotes The Jacob-Monad Model The Lac Operon (Inducible Operon): Jacob and Monad demonstrated how genes that code for enzymes that metabolize lactose are regulated

An operon consists of three elements: the genes that it controls a promotor region where RNA polymerase first binds an operator region between the promotor and the first gene which acts as an “on-off switch”.

Requires the production of 3 enzymes: Intestinal bacteria (E. coli) are able to absorb the disaccharide, lactose, and break and break it down to glucose and galactose (E. coli will only make these enzymes when grown in the presence of lactose) Requires the production of 3 enzymes:  - galactosidase – breaks down the lactose to glucose and galactose galactose permease – needed to transport lactose efficiently across bacterial cell membrane galactoside transacetylase – function is not clear

Structural genes of the lactose operon: Production of these enzymes is controlled by three structural genes and some closely linked DNA sequences responsible for controlling the structural genes – entire gene complex is called the operon Structural genes of the lactose operon: lacZ – codes for  - galactosidase lacY – codes for galactose permease lacA – codes for galactoside transacetylase

Next to the structural genes are 2 overlapping regulatory regions: promotor – region to which RNA polymerase binds to initiate transcription operator – region of DNA that acts as the switch that controls mRNA synthesis; sequence of bases that overlaps part of the promotor region

when lactose is absent, a repressor protein (in this case the lactose repressor) binds to the operator region – repressor protein is large enough to cover part of the promotor sequence, too, and blocks RNA polymerase from attaching to promotor – transcription is blocked when lactose is present, it acts as an inducer and “turns on” the transcription of the lactose operon lactose binds to repressor protein, inactivates it, and unblocks the promotor region allowing RNA polymerase to attach and begin transcription

Repressible and Inducible Operons: Two Types of Negative Gene Regulation A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription (trp operon) An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription (lac operon) Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor

Polypeptide subunits that make up enzymes for tryptophan synthesis Fig. 18-3a Tryptophan Operon - Repressible trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Regulatory gene Operator Start codon Stop codon 3 mRNA 5 mRNA RNA polymerase 5 E D C B A Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on By default the trp operon is on and the genes for tryptophan synthesis are transcribed

Fig. 18-3b-1 Tryptophan Operon DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off When tryptophan is present, it binds allosterically to regulatory repressor protein because it doesn’t need to make trp-building enzymes

Fig. 18-3b-2 Tryptophan Operon DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high

Gene Regulation in Eukaryotes most cells in a multicellular organism contain the same DNA but they don’t all use the DNA all the time individual cells express only a small fraction of their genes – those genes that are appropriate to the function of that particular cell type transcription of a cell’s DNA must be regulated factors such pregnancy may affect gene expression (genes for milk production are not used all the time) the environment may affect which genes are transcribed (length of day may increase a change in size of sex organs affecting the production of sex hormones in birds)

Gene expression may be regulated by: 1. the rate of transcription of genes the cell may regulate the transcription of individual genes through regulatory molecules (ex. steroids may stimulate the production of certain proteins) certain parts of eukaryotic chromosomes are in a highly condensed, compact state making it inaccessible to RNA polymerase some of these areas are structural and don’t contain genes other of these regions are functional genes that are not currently being transcribed

Histone Modifications and DNA Methylation Chemical modifications to histones play a direct role in the regulation of gene transcription The addition of acetyl groups to the histone “tails” opens up the chromatin promoting transcription by making genes more accessible Nucleosome Histone tails Unacetylated histones Acetylated histones

Histone Modification and DNA Methylation The addition of methyl groups (methylation) can condense chromatin and lead to reduced transcription DNA methylation is the addition of methyl groups to certain bases in DNA, usually cytosine although sometimes histone tails can be methylated Once methylated, genes usually remain so through successive cell divisions After replication, enzymes methylate the correct daughter strand so that the methylation pattern is inherited

In some cases methylation will inactivate entire chromosomes ex – Female mammals have two X chromosomes in each cell but only one is available for transcription – the other chromosome is condensed into a tight mass called a Barr body

Epigenetic Inheritance Though chromatin modifications do not alter DNA sequence, they may be passed to future generations of cells The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance Epigenetic modifications can be reversed, unlike mutations in DNA sequence 19

2. mRNAs may be translated at different rates mRNAs vary in stability (how long they last before they are degraded) and in the rate at which they are translated into protein 3. Proteins may require modification before they can carry out their functions in a cell 4. The rate of enzyme activity may be regulated (previously discussed in organic chemistry)