With the emergence of the nucleus as part of the eukaryotic cell plan comes the decoupling of transcription and translation. But something else is decoupled – the two processes that control the levels of messenger RNA in the cell. These two processes are mRNA synthesis, which occurs within the nucleus, and mRNA decay, which occurs in the cytoplasm. By physically separating the two processes, you run into a potential control problem where an increase in the synthesis of mRNA, for example, might not translate as an increase in mRNA levels because elevated mRNA decay rates in the cytoplasm might cancel out any increase in synthesis rates. And given that mRNA levels play important roles in embryological development, this could pose a serious problem for the evolutionary emergence of metazoa.
So is there a way to keep mRNA synthesis and decay rates coupled while allowing for the enhanced compartmentalization of the eukaryotic cells? Consider some recent research findings.
First of all, even though Rpb4/7 are part of the RNA polymerase, whose job is to synthesize RNA in the nucleus, it was determined that Rpb4/7 shuttle back and forth between the nucleus. Why in the world is a RNAP component spending time in the cytoplasm?
Secondly, it was determined that Rpb4/7 can bind the mRNA. However, it can only bind the mRNA if it is first part of the RNAP. Thus, the mRNA leaves the nucleus with Rpb4/7 stuck to it.
Third, out in the cytoplasm, Rpb4/7 moonlights and attract components of the RNA decay machinery.
In other words, the Rpb4/7 components that are not necessary for archaebacterial survival appear to play a key role in linking the processes of transcription and RNA decay in a eukaryotic context. For it is the location of Rpb4/7, whether in the nucleus or outside the nucleus, that determines their function. As researchers note:
This study provides an example for “conditional interaction” between two interacting partners that occurs only within specific molecular context. Such kind of interaction might be the basis for other cases of coupling between two processes. Summarily, conditional interaction between Rpb4/7 and mRNAs allows Pol II to impact not only transcription but also the fate of its products after they left the nucleus. This is the first indication that Pol II can affect mRNA decay in the cytoplasm and the first evidence for a direct mechanistic coupling between transcription in the nucleus and the two major mRNA decay processes in the cytoplasm. (emphasis added). 
And as I wrote in The Design Matrix:
The multifunctional nature of many proteins can be unlocked across time. A designer could implement one of the protein’s functions in service of unicellular life, while the other functions remain “in-waiting” for the appearance of the proper context for their expression. We know, for example, the multiple functions of various proteins are often unleashed as a function of location. A different function can be unlocked by localizing the protein in a different place in the cell, by localizing the protein outside of the cell, or by localizing the protein in a different type of cell.
1. Transcription in the nucleus and mRNA decay in the cytoplasm are coupled processes Vicky Goler-Baron, Michael Selitrennik, Oren Barkai, et al. Genes Dev. 2008 22: 2022-2027