Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 244th edition: p. 272-281(3rd ed: p. 280-289)• combinatorial control of transcription• inheritance of regulatory proteins• inheritance of chromatin condensationpatterns• inheritance of DNA methylation• induction of organ formationChapter 8: Control of Gene ExpressionCell memoryWhen a cell in a multicellular organism becomes committedto differentiate into a specific cell type, the terminal fate is normally maintained through all subsequent generations.• a signal (external or internal) triggers changes in theexpression of genes that direct the cell toward a particulardifferentiation pathway • these changes in gene expression must be remembered in progeny cells (= the cell must retain a memory of theseoriginal instructions)So what mechanisms create and maintain differential gene expression in specialized cell types?Combinatorial controls regulateeukaryotic gene expressionA group of transcription regulatory proteins work together tocontrol the expression of a single gene.• these transcription regulators bind to specific sequenceelements within the regulatory region of the gene, and together with general transcription factors andRNA polymerase form the transcription initiation complex• some of these proteins are activators while others arerepressors, and the sum of their effects determine thefinal level of expression of the gene• if any one of these proteins is subtracted from thetranscription initiation complex, transcription levels maybe significantly altered, because the complex would be incompleteTranscription Initiation ComplexThe expression of a single transcription regulator can switch on or off a gene or group of genesBecause the effect of transcription regulators iscombinatorial, the addition of a single proteincan complete a complex that switches a gene ON or OFF.• one protein could complete the transcription initiation complexes regulating several genes,thus switching several genes ON or OFF at onceExample: upon binding glucocorticoid hormone, the glucocorticoid receptor activates the transcription of several genes involved in the glucocorticoid response to stress/exerciseCombinatorial controls can create different cell typesAll the muscle-specific genes required for the specification ofskeletal muscle cells can be switched on by the action of asingle transcription regulator: MyoD• MyoD commits cells to become myoblasts • these cells proliferate and eventually fuse to form longmyotubes that become muscle fibers• muscle fibers express a number of muscle-specific products,giving it specific contractile and electrical propertiesIf MyoD is artificially expressed in fibroblasts fromconnective tissue, the cells are induced to behave as myoblasts, fusing to form muscle-like cellsCombinatorial control of the muscle cell fateOther cell types do not transcribe muscle-specific genes when MyoD is expressed in them.Why?Figure: fused, multi-nucleate fibroblast cells result from expression of MyoD (immunostained with antibodies that detect muscle-specific proteins).Fibroblasts and myoblasts are both derived from embryonic mesoderm, so both cell types contain some transcriptional regulators required for the induction of the muscle fate, and the addition of MyoD completes a unique combination. Other cell types that are not related to muscle cells have not accumulated the other required regulators.HematopoiesisDifferent combinations of a small number of transcriptionregulators can generate many different cell types.•A small number of transcription regulators in differentcombinations can control the expression of a largernumber of genes with shared regulatory DNA sequences•Each resulting cell type expresses different genes, asinduced by their particular combination of transcriptionregulators•Once a cell has differentiated into a particular cell type, it will remain differentiated, and all of its progeny will remain that cell typeCombinatorial controls can create different cell typesMechanisms of inheritance of patterns of gene expressionTerminally differentiated cell types give rise to cells like themselves when they divide. To preserve its identity, a cell must therefore remember the pattern of gene expression that led to its differentiation, and this memory must be passed to daughter cells.Mechanisms:1) positive feedback loop involving transcription regulators2) preservation of covalent histone modifications andchromatin condensation patterns3) preservation of DNA methylation patternsAll are examples of epigenetic inheritance, because they involve passing along enzymes and histones on top of DNA sequence information.Positive feedback loops and cell memoryOnce a transient signal triggers the expression of the transcription regulator (A), the regulator perpetuates its continued expression by activating its own transcription.• the regulator is passed on to daughter cells• transcription regulator A positively controls its own transcription, as well as the transcription of othercell type-specific genes, even if the signal that triggered its initial expression is no longer present in daughter cellsCell type-specificregulatorPositive feedback loops and cell memoryInheritance of histone modification patternsThe covalent modification of histones can result in chromatin condensation or decondensation, depending on which chemical groups are attached to the proteins, thus influencing local gene expression.• the tissue-specific histone-binding enzymes that catalyzeeach chemical modification recognize the siteswhere modifications have already occurred, then catalyzethe same modifications on nearby histonesThe replication of nucleosomesA model for the movement of replisomes past nucleosomes.The tissue-specific histone-binding enzyme is preservedthroughout cell division and passed on to daughter cells. The enzyme restores the parental pattern of histonemodification and chromatin condensation.Inheritance of histone modification patternsMethylation of cytosineAt specific CG sequences, cytosine can be methylated.These methylated sites turn off gene expression by attracting proteins that block transcription.Methylation of cytosine• after DNA is replicated, only the parental (template) strand is methylated• the enzyme maintenance methyltransferase recognizes already methylated CG