Regulation of Gene Expression Flashcards
Regulation of transcription in prokaryotes
If protein encoded by a gene is needed, the gene will be transcribed. If protein encoded by a gene is not needed, the gene will not be transcribed
Prokaryotic gene expression
DNA is not segregated in a nucleus, ribosomes can begin protein synthesis before mRNA is transcribed fully, control lies in determining which mRNAs are made
Operons
Polycistronic genes, related control sequences, one promotor regulates production of one mRNA, single RNA codes for several proteins in a pathway
Positive control of transcription
Requires a protein for transcription to occur
Negative control of transcription
Protein required to block transcription
Regulation of Lac operon
Negative control, repressor binds to operator- no transcription, inducer binds to repressor- transcription, decrease in inducer concentration- no transcription
Negative control of Lac operon
Repressor protein (lac I gene product), binds to operator region of Lac operon, prevents transcription from occuring
Function of inducer of Lac operon
Binds to repressor, prevents repressor from binding to operator, allows transcription, decrease in inducer concentration leaves repressor free to bind operator, no transcription
Repressors
Proteins produced by regulatory genes, normally bind to operator region of specific promoters, prevent mRNA transcription and protein production
Inducers
Small molecules that bind to repressors, repressor becomes inactive, leaves promoter, transcription and translation proceeds, repressor is bound in the absence of an inducer
Co-repressors
Some repressors are inactive on their own, require another molecule to bind before they bind to promoters and prevent transcription (co-repressors)
Catabolite repression of lac operon activity
Lac operon activity affected by glucose level, low glucose- high cAMP, activates cyclic AMP receptor protein (CRP), CRP at operator stimulates RNA polymerase binding
Regulation of RNA polymerase binding by sigma factors
Sigma factors bind RNA polymerase, sitmulate binding to certain sets of promoters, simultaneously activates transcription of several operons
Attenuation of transcription
Sequence and structure of mRNA can regulate gene expression, codons for Trp early in sequence, rate of translation influences RNA folding, low trp- genes expressed, high trp- no translation
Eukaryotic gene regulation
Much more complex, chromatin can be organized to allow or prevent transcription, different cell types have different regulatory patterns
Eukaryotic vs. prokaryotic gene expression
DNA in eukaryotes is organized into nucleosomes, operons are not present in eukaryotes, genes that encode proteins that function together are usually located on different chromosomes, transcription and translation are separated by intracellular compartmentation
Levels of regulation of eukaryotic gene expression
DNA and the chromosome, transcription, processing of transcripts, RNA transport and localization, initiation of translation, stability of mRNA, stability of protein
Regulation through histones and chromatin
Regulation by chromatin remodeling- displacement of nucleosomes from specific DNA sequences, histones tightly associated with chromatin may be modified to alter binding, acetylation changes charge and can regulate gene expression
Methylation of DNA
Cytosine residues in DNA can be methylated to produce 5-methylcytosine (5mC), methyl-cytosines are located in GC-rich sequences (CpG islands)- near promoter, can activate or repress transcription
Methylation and genetic disorders
Prader-Willi syndrome and Angelman syndrome, very different symptoms, from deletions of the same region of chromosome 15
Genome modifications
Some cells change their genomes- gene rearrangement, changes in gene copy number, chromosomal translocations, triplet repeats
Gene rearrangement
Segments of DNA can move from one location to another in genome
Example of gene rearrangement
Occurs in cells that produce antibodies, ordered rearrangement of B-lymphocyte variable gene regions coding for immunoglobulin chains, contributes to antibody diversity
VDJ recombination/somatic recombination
Mechanism of genetic recombination in immunoglobulin (B cells) and T cell receptor genes, takes place in primary lymphoid tissue, recombine variable regions to create large number of combinations, DNA spliced like mRNA
Gene amplification
Not the normal physiologic means of regulating gene expression in normal cells, regions of a chromosome undergo repeating cycles of DNA replication, new DNA forms double minutes, double minutes integrate into other chromosomes, amplify the gene
Double minutes
Small unstable chromosomes formed from newly synthesized DNA created during gene amplification
Gene deletions
Can occur through errors in DNA replication and cell division and are usually noticed only if a disease results, ex- loss of tumor suppressor gene results in cancer
Gene translocations
Ex- chronic myelogenous leukemia- Philadelphia chromosome, Burkitt’s lymphoma
Fragile X syndrome
Triplet repeat expansion, CGG triplet is amplified, 5’ of FMR-1 gene, 5-54 normal, 60-230 carrier, 230-4000 affected
Huntington disease
Triplet repeat expansion, CAG repeat in coding region, normal is 10-35 copies, disease is >35 repeats
Regulation at level of transcription
Promoters have core elements and regulatory elements (enhancers, silencers, hormone response elements), some proteins bind to elements, some to other proteins, external elements can influence binding
Regulation at transcription- steroid response
Hormone receptors have multiple domains, in cytosol bound to HSP, steroid binds, receptor dimerizes, nuclear localization signal (NLS) activated, migrate to nucleus, interact with transcription complex
Androgen insensitivity
Patients produce androgens, target cells fail to respond, lack the appropriate intracellular transcription factor receptors, leads to ambiguous genitalia or testicular feminization
Regulation at transcription- thyroxin and gene regulation
Thyroid hormone receptor in dimer with RXR on DNA, binds co-repressor with histone deacetylase, thyroxine changes conformation, coactivator with histone acetylase can bind
Thyroid hormone receptor disorders
Important in brain development in neonates, hypothyroidism increases T3 or TSH
DNA binding domains- zinc fingers
Estrogen receptor, one zinc ion is coordinated with four cysteine residues, alpha-helix with nucleotide recognition signal (NRS), NRS binds to a specific base sequence in the major groove of DNA
DNA binding domains- leucine-zipper
Alpha-helix of 30-40 amino acids, contains a leucine every seven amino acids, homo or hetero dimers, fos and jun genes
DNA binding domains- helix-turn-helix
One helix fits into the major groove of DNA, stable in binding the DNA without dimerization, homeodomain proteins
DNA binding domains- helix-loop-helix
Transcription factors, function as dimer (homo or hetero-dimers), myogenin in skeletal muscle
Regulation at transcription- regulatory cascades
One transcription factor can regulate many genes, may include another transcription factor, can regulate other sets of genes, one stimulus, many responses
Regulation at transcription- multiple sites in promoters
Promoters may have several transcription factor binding sites, different sites may direct activity in different circumstances
Regulation at mRNA processing- alternative splicing
Splicing can generate multiple mRNAs, may be tissue specific, can change function of the product, more than one poly A addition site in gene, use of site prevents splicing, alters carboxy terminus of protein
Regulation at mRNA processing- mRNA editing
mRNA sequence altered after transcription- rare, ex- ApoB100 gene, C deaminated to U, produces stop codon, shorter protein product
Regulation at translation- regulation of protein synthesis initiation by eIFs
Globin protein made only in presence of heme, heme inactivates kinase that inactivates eIF-2, globin mRNA translation can proceed, heme levels are high, eIF2 is not phosphorylated and is active
Regulation at translation- blocking translation
Ferritin synthesized when iron levels increase, ferritin mRNA has IRE, IRE-BP binds in absence of iron- no translation, increase in iron- translation occurs, ferritin binds excess iron
Regulation at translation- miRNAs
Small RNA molecules that regulate protein expression at translational level- induce degradation, repress translation
Actions of miRNAs
May be located within introns of target genes or organized in miRNA families, multiple target mRNAs, mRNAs bind multiple miRNAs, good potential drug targets
Regulation of stability of mRNA
Degradation rate of mRNA is important, transferrin receptor mRNA has 3’ IREs, low iron, bound IRE-BP protects 3’ ends, more protein made, more iron can enter the cell, high iron, no binding, mRNA degraded faster
Thalassemias
Defect in production of either alpha- or beta-globulin, tetramers form due to excess of other subunits, damage to RBCs, shorten lifespan of RBCs, cause anemia, splenomegaly common
