RNA Polymerase II (RNAPII) uniquely possesses an extended carboxy terminal domain (CTD) on its largest subunit, Rpb1, comprising a repetitive Tyr 1 Ser 2 Pro 3 Thr 4 Ser 5 Pro 6 Ser 7 motif with potential phosphorylation sites. The phosphorylation of the CTD serves as a signal for the binding of various transcription regulators for mRNA biogenesis including the mRNA capping complex. In eukaryotes, the 5 prime capping of the nascent transcript is the first detectable mRNA processing event, and is crucial for the productive transcript elongation. The binding of capping enzyme, RNA guanylyltransferases to the transcribing RNAPII is known to be primarily facilitated by the CTD, phosphorylated at Ser 5 (Ser 5P). Here we report that the Saccharomyces cerevesiae RNA guanylyltransferase (Ceg1) has dual specificity and interacts not only with Ser 5P but also with Ser 7P of the CTD. The Ser 7 of CTD is essential for the unconditional growth and efficient priming of the mRNA capping complex. The Arg159 and Arg185 of Ceg1 are the key residues that interact with the Ser 5P, while the Lys175 with Ser 7P of CTD. These interactions appear to be in a specific pattern of Ser 5P Ser 7P Ser 5P in a tri-heptad CTD (YSPTS P PS YSPTSPS P YSPTS P PS) and provide molecular insights into the Ceg1-CTD interaction for mRNA transcription.
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