Termination factor

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Termination is part of the process of transcribing mRNA.

In order for polymerase II (eukaryotes) or RNApol (bacteria) to stop transcribing at the end of a transcription unit, there must be certain signals available to the polymerase. While prokaryotic bacteria sometimes have no need for a termination factor (an inverted repeat followed by a string of A's on the template strand causes a phenomenon known as Rho-independent termination), most of the time some external signal is required.

In bacteria, proteins such as the Rho protein detect termination sequences in DNA that RNA polymerase cannot detect alone. In both Rho-dependent bacterial processes and eukaryotic processes, certain termination factors (proteins) including Rho use various methods to disengage polymerase once it reaches these regions. While Rho protein attaches itself to the DNA and "chases down" RNApol until it can simply knock it off, eukaryotic termination factors bind to the termination signal region and disturb RNApol as it moves by, causing it to fall off of the DNA strand within the next 300 bp's. This 300 bp region is removed during processing, before poly(A) tailing.