Termination of mRNA transcription is different in eukaryotes than in prokaryotes (see here or here). Whereas the prokaryotic RNA polymerase recognizes terminator signals, which sometimes function with the aid of the (rho) protein (see here), eukaryotic mRNA transcription proceeds as follows:

1. The eukaryotic RNA polymerase II usually continues to transcribe well past the end of the gene.

2. After the end of the gene has been reached, RNA golymerase II passes through one or more AATAAA sequences, which lie beyond the 3' end of the coding region (Figure 28.29).

3. The pre-mRNA, carrying this signal as AAUAAA, is then cleaved by a special endonuclease that recognizes the signal and cuts at a site 11 to 30 residues to its 3' side.

4. A tail of polyriboadenylic acid, poly(A), as much as 200 bases long, is added by a special non-template-directed polymerase. The function of the poly(A) tails of eukaryotic mRNAs is unknown. They cannot be essential for all messages, however, because some mRNAs (for example, most histone mRNAs in higher eukaryotes) do not have them. One idea is that they relate to message stability, because the tail-less messages typically have much shorter lifetimes in the nucleus.