Components of transcriptions factors
2. Prokaryotic Promoter
Prokaryotic promoter regions :-where sigma factors binds with RNA polymerasae to form holoenzymes.
- The -35 region
- The -10 ("Pribnow box") region comprise the core prokaryotic promoter
There are three enzymes are located adjacent to each other in the E. coli.This assortment
of genes and their regulatory regions is called the lac operon. The lac operon has the following DNA sequence elements. (a) Operator (lacO) – the binding site for repressor. (b) Promoter (lacP) – the binding site for RNA polymerase. (c) Repressor (lacI) gene – which encodes the Lac repressor protein. This protein binds to DNA at the operator and blocks transcription of the structural genes by RNA polymerase bound at the promoter. (d) Pi the promoter forlacI. (e) CAP binding site for cAMP/CAP complex. |
The activation of the lac operon in E. coli. In the absence of lactose, the lac
repressor binds to the operator (O) and prevents RNA polymerase bound to the promoter (P) from transcribing the structural genes. When lactose enters the cell, it binds to the lac repressor and causes a conformational change that inhibits its ability to bind DNA. |
Elongations:
On the other hand, after initiations of transcriptions. in elongations :
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Terminations
There is two terminations factors in transcriptions in prokaryotes:
a. Rho-independent transcription
termination involves terminator sequences within the RNA that signal the RNA polymerase to stop. The terminator sequence usually a Palindromic Sequences that forms a stem-loop hairpin structure that leads to the dissociation of the RNA polymerase from the DNA template. |
b. Rho-dependant terminations
uses a termination factor called ρ factor(rho factor) which is a protein to stop RNA synthesis at specific sites. • This protein binds at a rho utilization site on the RNA strand and runs along the mRNA towards the RNA polymerase . • A stem loop structure, upstream of the terminator region pause the RNA polymerase when the ρ factor reaches. Causing it to dissociate from DNA nad terminating transcriptions. |
Examples in E.coli
The simplest and most common type of termination signal in E. coli
consists of a symmetrical inverted repeat of a GC-rich sequence
followed by four or more A residues.
• Transcription of the GC-rich inverted repeat results in the formation of a
segment of RNA able to form a stable stem-loop by
complementary base pairing.