Lecture 20 - Transcriptional circuits in prokaryotes and eukaryotes Flashcards
Transcriptome (4)
Segment that is transcribed is called a transcriptome.
Most of the eukaryotic genome is never transcribed.
50% of prokaryote genome is transcribed.
Only a fraction of the transcriptome is transcribed at any one time.
Genes [5,3,4]
Type 1 = Abundant transcript - gives rise to many types of RNA - housekeeping genes, produce specific proteins in every cell type.e.g. glycolytic enzymes uses glucose to generate energy..
Type 2 = Not much protein is required -low levels required- rare transcript.
Type 3 = Found in diff amounts - code for tissue specific genes - e.g. highly abundant in cell 1 but not present in cell 2 - e.g. globin RBC but no other cell types.
Transcription requires recognition sequences in DNA that lie outside of the transcribed region (2)
Promoters - recruit RNA Polymerase to a DNA template.
RNA Polymerase works in one direction.
Recruitment of RNA Polmerase to promoters (5)
Eukaryotic/Prokaryotic RNA Polymerases do NOT MAKE stable contacts with DNA- they slide along without being able to recognise promoters.
Initiation factors recognise promoters.
Prokaryotes - Sigma factor.
Eukaryotes - TFII basal transcriptional machinery.
One stably recurited to DNA, RNA Polymerase able to convert from closed to open complex.
Prokaryotic promoters (5)
- The first nucleotide encoded into RNA is called +1 (initiation site) and then so on. Nucleotides before this are not copied into RNA and are numbered in negative numbers.
- In bacteria RNA polymerase does not bind to this sequence. This sigma factor binds and then RNA polymerase is recruited.
- Consensus sequence – a sequence of DNA having similar structure and function in different organisms. If you align all promoters there is a common sequence e.g. TATA (TATA box).
- The Pribnow box (also known as the Pribnow-Schaller box) is the sequence TATAAT of six nucleotides (thymine, adenine, thymine, etc.) that is an essential part of a promoter site on DNA for transcription to occur in bacteria.
Eukaryotic promoters (1)
TAT box can be seen upstream of start site this will recruit RNA polymerase II.
Regulatory transcription factors - RTF (6)
Regulatory changes are mediated by a diff type of TF, called RTF.
Ability of sigma factor to recruit RNA Polymerases to promoters are generic (occurs at all promoters).
Alter level of recruitment of RNA Polymerase and its ability to initiate transcription.
In eukaryotes they can influence local chromatin structure.
RF recognise their target sequence by intercating with DNA. Don’t have to unwind the DNA double helix to see target, an intact DNA molecule can present infomation to the cell.
Transcriptinal switches - Lac Operon
Lac ZYA - Genes needed to utilise lactose as a carbon source. 3 genes, transcribed as a single mRNA, all under control of one promoter.
Two regulators turn operon “on” and “off” in response to lactose/glucose levels, lac repressorand catabolite activator protein (CAP).
Lac repressor - Acts as a lactose sensor (indirectly through isomer allolactose). Blocks transcription of the operon but stops acting as repressor when lactose is present.
CAP - Acts as a glucose sensor (indirectly through ‘hunger signal’ molecule cAMP). Activates transcription of the operon,but only when glucose levels are low.
Operator is a negative regulatory site bound by lac repressor protein (operator and promoter bound by RNA Polymerase
Transcriptional switches - Lac Operon (6)
Lac ZYA - Genes needed to utilise lactose as a carbon source. 3 genes, transcribed as a single mRNA, all under control of one promoter.
Two regulators turn operon “on” and “off” in response to lactose/glucose levels, lac repressorand catabolite activator protein (CAP).
Lac repressor - Acts as a lactose sensor (indirectly through isomer allolactose). Blocks transcription of the operon but stops actingas repressor when lactose is present.
CAP - Acts as a glucose sensor (indirectly through ‘hunger signal’ molecule cAMP). Activates transcription of the operon,but only when glucose levels are low.
Operator is a negative regulatory site bound by lac repressor protein (operator and promoter bound by RNA Polymerase l
Transcriptional switches - Prokaryotic - Lac Operon - Low glucose (6)
Low glucose = cAMP production = cAMP + CAP = Binds to DNA. CAP helps RNA Polymerase bind to promoter = High levels of transcription.
Transcriptional switches - Prokaryotic - Lac Operon - Glucose and lactose present (5)
Low-level transcription of lac operon occurs.
Lac repressor released from the operator because the inducer (allolactose) is present.
cAMP levels low (as glucose is present).
CAP remains inactive, can’t bind to DNA = transcription occurs at a low level.
Lac repressor binds to promoter as glucose is present (preferred respiratory substrate).
Transcriptional switches - Eukaryotic (3)
o Oestrogen-responsive transcription
o Tissue-specific transcription (β-globin)
o A complex regulatory circuit (cell cycle)
Common regulatory elements - Trigger, Regulatory element, Transcription factor (12)
check document
Steroid hormone signalling (4)
Lipophilic - lipid soluble.
Pass through lipid component of cell membrane and bind to steroid hormone receptors.
Hormone-receptor complex formed acts as a transcription factor which facilitates/inhibits the transcription of a specific gene.
e.g. oestrogen/testosterone.
Eukaryotic RTF and hormones (4)
Eukaryotic RTFs activate transcription by stimulating recruitment of general TF and RNA Polymerase.
Oestrogen binds to oestrogen receptor complex which binds to TATA box.
Recruitment of TFII and RNA Polymerase II, leading to start of transcription.
Studying oestrogen leas to discovery of Tamoxifen for treatment of breast cancer. Acts as a competitive inhibitor. So, RNA polymerase II is not recruited and tumour growth is inhibited.
