Functional Coupling of Capping and Transcription of mRNA

In humans, 5' m(7)G cap addition is accomplished cotranscriptionally by the sequential action of the capping enzyme (Hce1) and the cap methyltransferase (Hcm1). We found that guanylylation and methylation occur efficiently during transcription with t(1/2)'s of less than 15 and 70 s, respectively. A two to four order of magnitude increase was found in the rate of guanylylation of RNA in transcription complexes compared to free RNA. This stimulation required only the RNA polymerase II elongation complex and Hce1. Capping activity was weakly associated with elongation but not preinitiation complexes. The CTD was not required for functional coupling but stimulated the rate of capping 4-fold. Inhibition of Cdk7 but not Cdk9 similarly slowed the rate of capping.