RNA polymerase II and the integration of nuclear events

The synthesis of a messenger RNA in the nucleus of a eukaryotic cell is an immensely complex undertaking. Each step in the pathway requires an enormous number of protein factors and identifying them and figuring out how they work has been a major goal of molecular biologists for the last two decades. Based on in vitro assays showing that each of the major steps, that is, transcription, capping, splicing, and polyadenylation, can be carried out in isolation, and because intuitively each of these reactions seemed quite distinct from the others, it had been widely assumed that the machinery responsible for each step was distinct and functioned essentially independently. However, numerous studies during the last few years have provided considerable evidence that this is not the case. In retrospect, this conclusion had been foreshadowed by earlier experiments pointing to the possibility that any one of these reactions could enhance some aspect of another. For example, evidence was presented consistent with the idea that the mRNA 5′ cap could play a role in allowing efficient transcription (Jove and Manley 1982), splicing (Edery and Sonenberg 1985), and even polyadenylation (Hart et al. 1985). Subsequently, it was shown in several labs that an intact polyadenylation signal could be required for transcription termination by RNA polymerase II (RNAP II) (Whitelaw and Proudfoot 1986; Logan et al. 1987; Connelly and Manley 1988), and that the presence of splicing signals on a pre-mRNA could enhance polyadenylation and vice versa (Niwa et al. 1990; Niwa and Berget 1991). However, none of these interactions really suggested just how intimate these associations might be, especially the emergence of RNAP II as an important component of all these reactions: capping, splicing, polyadenylation, as well as of course transcription.

The largest subunit of RNAP II has a unique domain, …