MESSENGER-RNA CAPPING ENZYME IS RECRUITED TO THE TRANSCRIPTION COMPLEX BY PHOSPHORYLATION OF THE RNA-POLYMERASE-II CARBOXY-TERMINAL DOMAIN

Capping of mRNA occurs shortly after transcription initiation, precedi ng other mRNA processing events such as mRNA splicing and polyadenylat ion. To determine the mechanism of coupling between transcription and capping, we tested for a physical interaction between capping enzyme a nd the transcription machinery. Capping enzyme is not stably associate d with basal transcription factors or the RNA polymerase II (Pol II) h oloenzyme. However, capping enzyme can directly and specifically inter act with the phosphorylated form of the RNA polymerase carboxy-termina l domain (CTD). This association occurs in the context of the transcri ption initiation complex and is blocked by the CTD-kinase inhibitor H8 . furthermore, conditional truncation mutants of the Pol II CTD are le thal when combined with a capping enzyme mutant. Our results provide i n vitro and in vivo evidence that capping enzyme is recruited to the t ranscription complex via phosphorylation of the RNA polymerase CTD.