Lecture 21 Flashcards
(17 cards)
Unique features of eukaryotic transcription
- Increase complex regulation of gene expression (3 types RNA poly., complex promoter elements)
- RNA processing (modifying 3’ and 5’ ends, splicing introns)
- Spatial and temporal separation between transcription (cytosol) and translation (nucleus)
RNA Poly I
Nucleolus. Involved in rRNA synthesis
RNA poly II
Nucleus. Makes mRNA and SnRNA (Small nucleolar RNA). Needs high levels.
RNA Poly III
Nucleus. Makes tRNA and SsRNA
Eukaryotic promoters
Recruit RNA Poly. to the start site. Always on the same DNA molecule as the gene they regulate (cis-acting element). Eukaryotic promoters differ in sequence and position relative to the regulated genes depending on whether they promote DNA poly 2 and/or 3. Enhancer sites may also be present.
empty
TATA box
located ~25 bases upstream of transcription start site and its highly constant. Mutation of a single base significantly decrease promoter activity. RNA Poly II promoter.
CAAT box and GC box
RNA Poly II promoter. Located between -40 and -150 bases upstream. Either on template or coding strand. Genes that are always expressed usually have GC box.
Initiator element
RNA Poly II promoter. Inr. Close to start site, centred at +1. Usually in combination with TATA box and so if it’s absent/degenerate, it can compensate for it. Defines start site as other promoter elements are distant from start site.
Downstream core promoter element
Centred +3 downstream of start site. Common in conjunction with inr that lack TATA box.
Enhancer sequences
DNA sequences that can stimulate transcription that cannot recruit RNA pol. Can exert stimulatory action over large distance. Can be anywhere. Helps establish specificity in cell types as they can stimulate the transcription of target genes when certain proteins are bound and the proteins may be expressed in certain cell types. Can be present on either DNA strand, cis-active.
TFIID
Binds to TATA box causing configurational change in DNA strand where it will open minor groove to increase hydrophobic interactions between the transcription factor and DNA. Once TFIID is secure in promoter element, it changes protein shape allowing more TF to be recruited. They bind to docking site in TFIID which recruit even more. When all TFII are recruited, they recruit RNA Pol. 2 and unwind DNA and phosphorylate RNA Pol. 2 shifting it to elongation.
Transcription factors
Proteins that bind to promoter/enhancer elements that helps recruit RNA poly. Trans-acting elements as the gene that codes for a particular TF may not always be located on the same DNA molecule as the gene thats regulating. TFII recruits RNA pol. where subset is TFIIA, TFIIB etc.
Transcription machinery
Whole collection of TFs and RNA poly. Needed for transcription = basal transcription complex. Transcripts at low level. To increase transcription rate, increase TFs bound to enhancer elements. TFs can recruit other proteins (mediators) to activate/repress transcription. They can be tissue-specific and can have different roles depending on what mediator is present. Combinatorial control of TFs forms basis of functional complexity of eukaryotes.
Oestrogen
Steroid hormone that regulates ovarian cycle. Diffuse into cell and bind to nuclear receptors in the cytoplasm/nucleoplasm.
Hormone control of gene expression
The nuclear hormone receptor has ligand-binding and DNA-binding domains (characterised by Zn fingers). Zn fingers can bind to specific DNA sequences. Ligand-binding domain allows hydrophobic pocket of protein to bind. Receptor-ligand complex modifies expression of specific genes by recruiting coactivators/corepressors
Hormone receptors are drug targets
Antagonists of oestrogen receptors stop oestrogen-mediated cell growth and are used to treat some breast-cancers. Other pathways e.g androgen receptor stimulate expression of genes (agonists) expression of genes that enhance development of lean muscle mass.
