Transcriptional Circuits in Prokaryotes Flashcards
What is a transcriptome?
It is the segment of a genome that is transcribed.
What are the 4 levels of gene transcription?
- Transcribed abundantly (if on in every cell, called a housekeeping gene)
- Transcribed rarely
- Transcribed only in certain tissue cells (tissue-specific)
- Transcription is induced via a stimulus (turning it from a no/rare transcript to an abundant transcript)
What are enhancers?
They are sequences of DNA (not immediately adjacent to where transcription starts) that act to enhance the recruitment of RNA polymerase to a promoter. They can reside in the 5’, 3’ or even the introns. They are very strong binding sites for specificity factors.
Expand on the stability of recruitment of RNA Polymerase II to promoters.
In both eukaryotes and prokaryotes, RNA polymerases can’t make stable contacts with DNA, they simply slide along. Once stably recruited, the RNA polymerases convert from a closed complex to an open complex.
The recognition of promoters is mediated by initiation factors. What are these factors in prokaryotes?
It is the sigma factor, which recognises the -35 and -10 motifs common to prokaryotic promoters.
The recognition of promoters is mediated by initiation factors. What are these factors in eukaryotes?
It is the TF2 basal transcriptional machinery (TF2A, TF2B, etc.).
What is the function of regulatory transcription factors?
They function to dramatically alter the level of recruitment of RNA polymerase and/or its ability to initiate transcription.
List the known transcriptional switch in prokaryotes (1) and eukaryotes (3).
PROKARYOTES:
- the lac operon
EUKARYOTES:
- oestrogen-responsive transcription
- tissue-specific translation (beta-globin)
- a complex regulatory circuit (cell cycle)
Describe the lac operon.
It occurs in prokaryotes. It is activated when lactose is available (and glucose is not). Its function is to use lactose as an energy supply.
There are 3 genes associated with this: lacZ, lacY and lacA. LacZ encodes the enzyme beta-galactosidase, which breaks lactose down to its monomers, galactose and glucose. LacY encodes the protein lactose permease, which is a transmembrane ‘pump’ that allows the cell to import lactose. LacA encodes an enzyme known as transacetylase that attaches a particular chemical group to target molecules. (knowledge of the genes is extra info)
There is a CAP site before the promoter (site of RNA polymerase binding), and an OPERATOR site after it.
The CAP site is a positive regulatory site (when CAP bound, promotes transcription) bound by the catabolite activator protein.
The operator site is a negative regulatory site (when lac repressor bound, prevents transcription) bound by the lac repressor protein.
Can an intact DNA molecule present information to the cell, and why?
Yes, it can, because regulatory factors can recognise their target sequences by interacting with the DNA; the DNA double helix doesn’t need to be unwound.
Describe oestrogen-responsive transcription.
An oestrogen molecule binds to a free oestrogen receptor complex, which binds to the oestrogen-responsive element on the mRNA. This stimulates the recruitment of general transcription factors and RNA polymerase.
Tamoxifen is an antagonist of oestrogen-responsive transcription. Its binding to the oestrogen-responsive element will prevent transcription.
Describe tissue-specific transcription (specifically beta-globin).
It has a beta-globin specific transcription factor, GATA-1, which makes the cell lose its nucleus, etc.
Explain the regulation of the cell cycle.
At G0, cells are at a resting state, waiting for a mitogenic signal to get them into G1. The point at G1 at which commitment occurs and the cell no longer requires growth factors to complete the cell cycle has been termed the ‘restriction point’. After that, the cell is insensitive to the external environment until the cell cycle is complete.
Describe the role of transcription factors in the cell cycle.
The promoters for G1/S transition genes are activated by a factor called E2F. This gene is repressed (in G0 and early G1) by the product of the Retinoblastoma gene (pRB). When the mitogenic signal is given, Cdks phosphorylate the pRBs, freeing the E2F, allowing transition to start.
What regulates the transitions in the cell cycle?
Cyclin-dependant kinases (Cdk)
