Gene expression CHAPTER 18 Bacterial Gene Regulation Bacteria

Gene expression CHAPTER 18

Bacterial Gene Regulation Bacteria regulate transcription based upon environmental conditions E. coli depends on the eating habits of its host If the host doesn’t eat foods with tryptophan-bacteria activates metabolic pathway Produces tryptohan from another source Gene regulation takes place via an operon Key components Operator – the switch that controls on/off Promoter – the attachment site for RNA polymerase Enzyme coding genes

Bacterial Gene Regulation trp Operon Is turned “on” in the absence of tryptophan Is turned off by a repressor protein – trp repressor (trp. R) Blocks RNA polymerase from binding to the promoter When tryptophan is present it binds to the repressor protein and blocks transcription Prevents the enzymes that will synthesize tryptophan from other sources This is called a repressible operon Usually on but can be turned off by a particular molecule

Bacterial Gene Regulation lac operon Usually off, but turned on by the presence of a particular molecule Β-galactosidase breaks down lactose into glucose and galactose Without lactose, small amount of β- galactosidase produced Genes for Β-galactosidase are located in the lac operon Key componenets lac I gene – regulatory gene that codes for a repressor Inducer – small molecule that inactivates the repressor This is called an inducible operon The signal to start is induced by a molecule (lactose)

Bacterial Gene Regulation Positive Gene Regulation E. coli prefers glucose to lactose when both present Only with metabolize lactose when glucose in short demand c. AMP (cyclic AMP) – accumulates when glucose is scarce Involves an activator – protein that binds to DNA and causes transcription (catabolite activator protein) As c. AMP increases, it binds to CAP This binds to promoter site upstream from lac promoter Increases the affinity for RNA polymerase As c. AMP decreases (glucose increases) CAP detaches from promoter

Eukaryotic Gene Expression Chromosome structure can affect gene expression Modification to histone structure Histone acetylation Acetyl groups are attached to histone tails Detach from neighboring nucleosomes Easier for transcription proteins to access genes in aceltylated areas

Eukaryotic Gene Expression Gene expression can be regulated by changing DNA More or less affinity for transcription proteins (factors) Can use Enhancers and Transcriptions factors Enhancers can contain activators or repressors Activator proteins bind to a distal control element in an enhancer DNA bending protein brings the activators closer to the promoter Activators bind to mediator proteins and transcription factors Transcription initiation complex forms and transcription begins

Eukaryotic Gene Expression Combinatorial control All cells have the same DNA for coding Differing control elements can control for gene expression in different cells Turning genes on and off at the same time Genes for enzymes of a metabolic pathway are typically on different chromosomes Coordinated control usually occurs in response to external signals Ex. Hormone entering a cell Every gene in the pathway will respond to the signal Some non-hormal signals bind to signal transduction pathways

Eukaryotic Gene Expression New research has shown that DNA areas may loop into areas that are high in RNA polymerases in the nucleus Chromosomes may interact with one another Genes may also be relocated to transcription factories in the nucleus to ready for transcription

Eukaryotic Gene Expression Transcription does not mean gene expression – amount of functional protein determines expression Alternative RNA Spicing Different m. RNA molecules are produced from the same transcript Can expand the number of genes available m. RNA Degradation Initiation of Translation The amount of time m. RNA is available can determine protein output Gene regulation can occur at the initiation of translation which can have an affect on gene expression Protein Processing and Degradation Proteins must be folded properly to function Proteins have a designated amount of time they function within the cell before destruction

Eukaryotic Gene Expression Micro. RNA’s and Small Interfering RNA’s mi. RNA’s – small single stranded RNA molecules Long sequences that fold back on themselves Form hairpin turns – cut away Dicer cuts into short fragments Forms a complex with proteins that can bind to m. RNA Either degrades the m. RNA or blocks translation si. RNA’s – similar in function to mi. RNAs but have a longer strand

Cell Differentiation What determines how the first cells will differentiate? Egg cytoplasm contains RNA’s and proteins – cytoplasmic determinants After the first division the two cells are exposed to different determinants These determine which genes will be expressed in the cell Cell environment – other major factor in embryonic gene expression Induction – contact of cell to cell surface molecules Binding of growth factors by neighboring cells

Cell Differentiation Before development of organs, the body position must be determined Cytoplasmic determinant and inductive signals contribute to spatial organization Forms the axes of the body Positional cues are cytoplasmic determinants and inductive signals – tell a cell its location relative to body axes and neighboring cells

Cell Differentiation Homeotic genes (Hox genes) In fruit flies, genes that control pattern formation late in embryo, larva, and adult development Discovered by Edward B. Lewis by studying mutant fruit flies with abnormal patterns of development

Development of Cancer The same systems important during development are prominent in cancer cells Also have cancer causing genes – oncogenes Normal genes are referred to as proto-oncogenes Conversion to an oncogene – 3 ways: Movement of DNA within the genome Amplification Point of proto-oncogene mutations in a control element of an proto-oncogene

Development of Cancer Interference with cell signaling pathways ras gene – proto-oncogene Mutated in about 30% of human cancers G-protein – relays a signal from a growth factor Result is to stimulate the cell cycles Mutations result in a hyperactive Ras protein in the absence of the growth factor – increased cell division p 53 gene – tumor suppressor gene Mutated in 50% of human cancers Called the guardian angel of the genome-activates many other genes Codes for cell cycle inhibiting proteins Leads to excessive cell growth

Development of Cancer More than one somatic cell mutation is needed for production of a cancer cell This is why cancer risk increases with age Ex. Colorectal cancer Starts as a normal benign polyp but divides frequently May become malignant-also accumulates mutations converting proto-oncogenes to oncogenes and knocks out tumor suppressor genes Cells become cancerous About half a dozen changes needed at the DNA level for a cell to become cncersours

Development of Cancer Inheritance of oncogenes or mutated tumor suppressor genes increases risk 5 -10% breast cancer have a strong inheritance basis Mutations to BRCA 1 or BRCA 2 are found in at least half inherited cancers BRCA 1 mutation has a 60% probability of developing breast cancer before age 50 (only 2% if no mutation) BRCA genes are considered tumor suppressor genes Both protein products function in DNA repair Cancer can also be caused by viruses Ex. Epstein-Barr virus (Lymphomas), Papillomaviruses (cervical cancer) Viruses play a role in about 15% of all cancers