Chapter 37 Flashcards
Differential Gene Regulation
Multicellular organisms use differential gene regulation to generate different cell types
Gene expression in eukaryotes is influenced by three important characteristics:
- More complex transcriptional regulation
- RNA processing, including extensive processing of mRNA precursors
- The nuclear membrane, which separates the site of RNA synthesis from that of protein synthesis
Transcription involves:
- Initiation
- Elongation
- Termination
RNA synthesis = end product of transcription, which takes place in nucleus
Protein synthesis = end product of translation, which takes place in the cytoplasm
RNA Polymerases – 3 Types in Eukaryotic Cells
All polymerases are similar in structure, but RNA polymerase II has a unique domain, called carboxyl-terminal domain (plays important regulatory role)
Eukaryotic promoters (cis-acting elements) are more complicated than bacterial promoters
- Each polymerase has distinct promoters
Promoters bind to proteins, called trans-acting elements or transcription factors, that regulate polymerase activity
RNA polymerase I:
located in nucleoli, transcribes genes for rRNA
RNA polymerase II:
catalyst for mRNA synthesis, is controlled by a wide array of promoters, including the TATA box and enhancers
RNA polymerase III:
responds to promoters in the genes to be transcribed, such as those encoding tRNA and 5s ribosomal RNA
True or False: A variety of small RNAs play important roles in processing ad regulating the products of the three polymerases
True
Common promoters for RNA polymerase II include:
- The upstream TATA box, which is located around -25 bp upstream of the initiation site
- The initiator element (Inr) located around +1, is often paired w/ the TATA box
- The downstream core promoter element (DPE), located around +30, woks in cooperation w/ Inr when the TATA box is absent
- Other regulatory elements are the CAAAT box and GC box, located between -40 and -150 bp upstream
- The GC box is common in genes that are continuously expressed
- The CAAT box and the GC box can be located on either strand of the DNA
Transcription Factors
TFs: proteins that bind to promoters to regulate gene expression
Transcription factors that guide RNA polymerase II: TFIIA, TFIIB, TFIID, TFIIE, TFIIF
TATA-box binding protein (TBP) is a component of TFIID
- Binds to the TATA box
- Nucleating the formation of the preinitiation complex (PIC)
TFIIH: an ATP-dependent helicase that unwinds the DNA in order for transcription to occurs
- Phosphorylates the carboxy-terminal domain of the polymerase
- Facilitating the transition from initiation phase to the elongation phase
Enhancer Sequences
Enhancer sequences are cis-acting elements that have no promoter activity but can stimulate the effectiveness of promoters even when located thousands of nucleotides from the start site
Enhancers operate in conjunction w/ specific enhancer-binding proteins called transcription activators
Gene Expression – Regulation by Hormones
Nuclear hormone receptors bind to specific regions of the DNA called response elements
- Ex. The estrogen receptor binds to the estrogen response element (ERE)
Nuclear hormone receptors have group highly conserved domains:
- The DNA binding domain: towards the center of the primary structures
- Characterized by zinc-finger domains that confer specific DNA binding - The ligand binding domain: contains an activation domain
- Lies toward the carboxyl terminus of the primary structure - The amino terminal activation domain
- Enables the receptor to interact w/ other proteins - The hinge domain
- Contains a nuclear localization signal
Nuclear hormone receptors can alter transcription in two general ways
1. Co-activators can interact w/ the receptor ~ ligand complex and stimulate transcription
2. Co-repressors can bind to receptors and inhibit transcription
- Upon ligand-receptor binding, the repression is relieved, and transcription occurs
