Pre-mRNA Processing and the CTD of RNA Polymerase II: The Tail That Wags the Dog?

spliced and polyadenylated mRNA, and pre-mRNA tranEukaryotic cells must execute a complex program to scripts engineered to be synthesized by RNA polymergenerate mature functional messenger RNA (mRNA). ase III in mammalian cells fail to be spliced (Sisodia et After the transcription of a nuclear gene by RNA polyal., 1987). Greenleaf (1993) proposed that the negative merase II (RNA Pol II) is initiated, introns must be recharge of the hyperphosphorylated CTD on RNA Pol moved from the pre-mRNA and the mature 39 end of IIo might facilitate direct electrostatic interactions with the transcript defined by specific cleavage and polyadehighly basic arginine–serine (RS) dipeptide repeat senylation. Although the splicing and 39-end formation quences characteristic of many splicing factors in multireactions can occur in isolation from transcription in cellular eukaryotes, including members of theSR protein vitro, there have been periodic reminders that transcripfamily (reviewed by Fu, 1995). This prediction has proven tion and pre-mRNA processing may be coupled in vivo. to be very close to the mark. Electron microscopy and microdissection studies illusAssociation of Pre-mRNA Splicing Factors trate that splicing can occur cotranscriptionally (Beyer with the CTD and Osheim, 1988; Wetterberg et al., 1996), and available Several groups have now established that SR proteins evidence indicates that 39-end processing not only preand other spliceosome components are indeed physicedes but also may be a prerequisite for termination of cally associated with RNA polymerase II. Antibodies transcription (Connelly and Manley, 1988; Russo and specific for the RNA Pol II CTD have been found to Sherman, 1989). However, factors or mechanisms that coimmunoprecipitate from mammalian cell extracts a