Chapter 28 – Regulation of Gene Expression- The cellular concentration of a protein is determined by a delicate balance of at least seven processes:o Synthesis of the primary RNA transcripto Posttranscriptional modification of mRNAo Messenger RNA degradationo Protein synthesis (translation)o Posttranslational modification of proteinso Protein targeting and transporto Protein degradation28.1 Principles of Gene Regulation- Housekeeping genes – genes for products that are required at all times and are expressed at a more or less constant level in virtually every cell of a species or organism- Constitutive gene expression – unvarying expression of a gene- Regulated gene expression – when cellular levels of gene products rise and fall in response to molecular signals- Inducible – gene products that increase in concentration under particular molecular circumstanceso Induction – the process of increasing the expression of these genes- Repressible – gene products that decrease in concentration in response to a molecular signalo Repression – the process of decreasing the expression of these genes- Transcription is regulated by protein-DNA interactionsRNA Polymerase Binds to DNA at Promoters- The regulation of transcription initiation often entails changes in how RNA polymerase interacts with a promoter- Differences in the promoter sequence may affect the frequency of transcription initiationo Mutations that result in a shift away from the consensus sequence usually decrease promoter functiono Mutations towards consensus enhance promoter function- The RNA polymerase-promoter interaction strongly influences the rate of transcription initiation for housekeeping genesTranscription Initiation Is Regulated by Proteins That Bind to or near Promoters- Three types of proteins regulate transcription initiation by RNA polymerase:o Specificity factors – alter the specificity of RNA polymerase for a givenpromoter or set of promoterso Repressors – impede access of RNA polymerase to the promotero Activators – enhance the RNA polymerase-promoter interaction- The subunit of the E. coli RNA polymerase holoenzyme is a specificity factor that mediates promoter recognition and bindingo 70 is the most commono 32 is used during heat stress- Repressors bind to specific sites on the DNAo Operators – such binding sites in bacterial cells that are generally neara promotero Blocks binding or movement of RNA polymeraseo Negative regulation – regulation by means of a repressor protein that blocks transcriptiono Effector – a molecular signal that regulates repressor binding Usually a small molecule or protein that binds to the repressor,causing a conformational change- Positive regulation - activators bind to DNA and enhance the activity of RNA polymerase at a promotero Particularly common in eukaryotesMany Bacterial Genes Are Clustered and Regulated in Operons- Many bacterial mRNAs are polycistronic, meaning they have multiple genes on a single transcript and require one promoter, which is the site for regulation of all the genes in the clustero Operon – the gene cluster and promoter, plus additional sequences that function together in regulationThe lac Operon Is Subject to Negative Regulation- The lac operon contains genes for:o-galactosidase (Z)o Galactoside permease (Y)o Thiogalactoside transacetylase (A)- In the absence of lactose, the lac operon genes are repressed- In higher eukaryotes, almost all protein-encoding genes are transcribed separatelyRegulatory Proteins Have Discrete DNA-Binding Domains- To bind specifically to DNA sequences, regulatory proteins must recognize surface features on the DNA- Most of the protein-DNA contacts that impart specificity are hydrogen bonds- DNA-binding sites for regulatory proteins are often inverted repeats of a short DNA sequence- Helix-turn-helix o Crucial to the interaction of many bacterial regulatory proteins with DNAo Comprises about 20 amino acids in two short -helical segments separated by a turn One of the two helices is called the recognition helix because it usually contains many of the amino acids that interact with the DNA in a sequence-specific way The recognition helix is positioned in or nearly in the major grooveo The Lac repressor has this motif- Zinc Finger o 30 amino acids form an elongated loop held together at the base by a Zn2+ ion, which stabilizes the motifo Weak interaction with DNA for a single Zn fingero Usually in eukaryotes- Homeodomain o A domain of 60 amino acids that is seen as a DNA-binding domain in transcriptional regulators Homeobox – the DNA sequence that encodes this domainRegulatory Proteins Also Have Protein-Protein Interaction Domains- Leucine Zipper o An amphipathic helix with a series of hydrophobic amino acid residues concentrated on one sideo The hydrophobic surface forms the area of contact between the two polypeptides of a dimer- Basic Helix-Loop-Helix o A conserved region of about 50 amino acids that can form two short amphipathic helices linked by a loop of variable lengtho The helix-loop-helix motifs of two polypeptides interact to form dimers- Subunit Mixing in Eukaryotic Regulatory Proteins o Three types of additional domains for protein-protein interaction: Glutamine-rich domains Proline-rich domains Acidic domains28.2 Regulation of Gene Expression in BacteriaThe lac Operon Undergoes Positive Regulation- The availability of glucose also affects the expression of the lac genes- Catabolite repression – a regulatory mechanism that restricts expression of the genes required for catabolism of lactose, arabinose, and other sugars in the presence of glucose, even when these secondary sugars are also present- cAMP receptor protein (CRP) – a homodimer with binding sites for DNA and cAMPo Mediates the effect of glucose in the lac operono Has a helix-turn-helix motif in its DNA-binding domaino A positive regulatory element responsive to glucose levels- As [cAMP] declines, CRP binding to DNA declines, thereby decreasing the expression of the lac operono In the presence of glucose, synthesis of [cAMP] is inhibitedo Strong induction of the lac operon therefore requires both lactose to inactivate the lac repressor and a lowered concentration of glucose- Regulon – a network of operons with a common regulatorMany Genes for Amino Acid Biosynthetic Enzymes Are Regulated by Transcription
View Full Document
Unlocking...