mRNA Capping Enzyme Activity Is Coupled to an Early Transcription Elongation

The eukaryotic mRNAs produced by RNA polymerase II (Pol II) are capped with an inverted 7-methyl-guanosine (m7G) linked to the first residue of the mRNA (36). This event occurs by a series of three enzymatic reactions; The 5′ triphosphate end of the nascent RNA Pol II transcript is cleaved by 5′ RNA triphosphatase to produce the diphosphate end. RNA guanylyltransferase forms a covalent enzyme-GMP complex and subsequently caps the RNA substrate by adding a guanosine residue in a 5′-5′ triphosphate linkage. The cap is then methylated by RNA (guanine-7) methyltransferase (23, 39). In higher eukaryotes, a bifunctional monomeric polypeptide carries both the RNA triphosphatase and guanylyltransferase activities, while the capping enzyme from yeast is a complex of RNA triphosphatase and guanylyltransferase subunits (40). The polypeptides are encoded by the CET1 and CEG1 genes, respectively in Saccharomyces cerevisiae, and both are essential for the cell viability. Capping, the first mRNA modification, occurs by the time the transcript is only 25 to 30 nucleotides long in an early transcription phase. Such cotranscriptional capping is mediated by recruitment of capping enzyme machinery to the phosphorylated carboxy-terminal domain (CTD) of the largest subunit of Pol II (7, 15, 22, 51). The CTD of Pol II has an unusual structure with many heptapeptide repeats (YSPTSPS). The capping enzyme binds directly and specifically to the phosphorylated CTD of Pol II via the Ceg1 subunit (yeast) or the guanylyltransferase domain (metazoan). Furthermore, the guanylyltransferase activity of Ceg1 associated with phosphorylated CTD is allosterically regulated by both the Cet1 and phosphorylated CTD to ensure it has a coordinated capping activity (8). The mammalian capping enzyme is also allosterically regulated by an interaction with the phosphorylated CTD (16, 25). The CTD phosphorylated at serine 5 of the heptapeptide repeat appears to be important for the capping reaction because it stimulates the guanylyltransferase activity, although both serine 2 phosphorylation and serine 5 phosphorylation can mediate the protein interaction. An interesting extension to these findings was added by a chromatin immunoprecipitation assay, which provides in vivo evidence that the capping enzyme machinery interacts dynamically with Pol II during a transcription cycle (20, 35). The capping enzyme interacts with Pol II immediately after the serine 5 of CTD is phosphorylated. As serine 5 phosphorylation decreases in an early elongation phase, the capping enzyme dissociates from the transcription complex. Recently, data from several groups have raised the concept of “checkpoints” in transcription, especially in an early phase (summarized in reference 26). As operated during the cell cycle to ensure that each phase of the cycle is complete before the next one begins, checkpoint in early transcription is suggested to play a role in ensuring that only the properly modified RNA at the 5′ end is extended. The Pol II transcription is thus subjected to checkpoint control for the coordinated transcription with mRNA capping (6, 25, 26). Several transcription factors have been reported to play in this window. DSIF, a human homolog of the yeast transcription factor Spt4/Spt5, increases the pausing of Pol II and thus plays a role as a negative factor (42, 46, 49). Within this temporal and spatial interval, while Pol II with the hypophosphorylated CTD is paused at the promoter-proximal region, many factors are intended to target the capping enzymes to increase their recruitment or to enhance their catalytic activities. In addition to TFIIH, which creates a binding epitope for the capping enzyme on CTD by serine 5 phosphorylation, as described above, Spt5 itself interacts with the triphosphatase and guanylyltransferase components of the capping enzyme (21, 27, 44). In the case of human immunodeficiency virus (HIV), DSIF/Spt5-induced transcription arrest allows HIV-encoded Tat to interact with capping enzymes and to stimulate their catalytic activities (5, 6). Phosphorylation of Pol II CTD is critical for the transition to the elongation phase. At this step, the elongation factor P-TEFb, a DRB-sensitive protein kinase, phosphorylates the Pol II CTD and Spt5 (19, 29, 38). HIV Tat also interacts with P-TEFb in this step (29). In S. cerevisae, Ctk1 kinase complex and the Bur1 kinase complex facilitate the transcription elongation (18, 50). The phosphorylation of Pol II CTD is thus meant to lead to the formation of the processive transcription elongation complexes. According to recent reports, the capping enzyme in Schizosaccharomyces pombe interacts with Cdk9/Pch1, a yeast P-TEFb homolog (28). How does the capping enzyme fit into the complicated scheme to delineate it in the order of pausing, capping, and the reversion of pausing in the checkpoint model? If the elongation-competent transition does not occur until the RNA is capped and the capping is the major determinant to shift the Pol II status, it could be indicative that a capping enzyme plays a critical role in regulation of an early transcription in addition to its role in simple cap formation. To study whether the capping enzyme plays a key role in coordinating mRNA processing and transcription elongation, we used a well-characterized yeast system. Because Ceg1, a capping enzyme subunit, contributes to transcription via its typical cap formation activity, this study examined various ceg1 temperature-sensitive alleles to determine if there is any additional role in transcription elongation. Among them ceg1-63 displayed 6-azauracil (6AU) sensitivity and a defect in PUR5 induction by 6AU treatment. This study shows that transcription through the pause sites artificially inserted at the promoter-proximal region was severely inhibited in ceg1-63. We show that such an elongation defect was coupled to the reduced guanylyltransferase activity. However, it happened independently of the turnover of uncapped transcripts. These results indicate that the transcription ternary complexes are held at the promoter as long as their RNAs have not been properly capped. That is, capping enzyme plays a critical role in the promoter-proximal checkpoint window by reinforcing the checkpoint security circuit and probably by reversing the transcription arrest in time. This finding also strongly supports the transcription checkpoint model, in which an early transcription is tightly regulated.