Content:
• Introduction to Transcription Termination
• Rho Factor (ρ)
• Rho Dependent Termination
• Rho Independent Termination
Introduction to Transcription Termination
In prokaryotes there are two types of termination that may occur. These are Rho Dependent Termination and Rho Independent Termination. Termination is controlled by specific nucleotide sequences called terminator sequences. These sequences are defined as points where the rate of addition of the next RNA nucleotide is slower than the rate of nucleotide release.
Rho Factor (ρ)
Introduction
Rho (Greek lettering - ρ) factor is an essential prokaryotic protein. Rho forms a hexameric ring structure with an open configuration. It contains six subunit laterally organised with a distance of 12 Angstroms at the opening. This is to accommodate the single stranded RNA molecule. In E. coli, Rho is a 274.6 kD hexamer with identical subunits. It also has 419 amino acid residues. (2)
Rho Structure
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In this prokaryotic specific mechanism, termination occurs by the formation of a stem loop hairpin structure which occurs at a palindrome.
Palindrome
A palindromic sequence occurs where a series of base pairs in the 5' to 3' direction on the DNA or RNA strand is repeated in the inverse sequence further along the same strand. This is the termination signal for the Rho Independent Termination. These sequences are able to base pair together as the RNA strand folds over on itself to form a stem loop structure as it is being transcribes. The sequence of the termination signal is followed by a series of Adenines which transcribes to a poly-Uracil tail on RNA. (1)
Stem Loop Structure
The stem loop hairpin structure is typically 7 to 20 base pairs long and is rich in Guanine-Cytosine bonding. These base pairs are connected by three hydrogen bonds giving strength and stability to the RNA duplex. (4)
Termination