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Capping enzyme
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<p>A capping enzyme (CE) is an <a href="/facts/Enzyme/DSI1Fh7y">enzyme</a> that <a href="/facts/Catalysis/LPBS7TQC">catalyzes</a> the attachment of the <a href="/facts/5%2527_cap/cWUeEBJ6">5' cap</a> to <a href="/facts/Messenger_RNA/oUL6qxQn">messenger RNA</a> molecules that are in the process of being synthesized in the <a href="/facts/Cell_nucleus/D7sFRa3c">cell nucleus</a> during the first stages of <a href="/facts/Gene_expression/alCPohLR">gene expression</a>. The addition of the cap occurs <a href="/facts/Transcription_(genetics)/bDgp9HDN">co-transcriptionally</a>, after the growing <a href="/facts/RNA/HxPeNfNj">RNA</a> molecule contains as little as 25 <a href="/facts/Nucleotide/pQKEwlJI">nucleotides</a>. The enzymatic reaction is catalyzed specifically by the <a href="/facts/Phosphorylation/LXUEEeIL">phosphorylated</a> carboxyl-terminal domain (CTD) of <a href="/facts/RNA_polymerase_II/MK2kgybw">RNA polymerase II</a>. The 5' cap is therefore specific to RNAs synthesized by this polymerase rather than those synthesized by <a href="/facts/RNA_polymerase_I/ACQYJH2M">RNA polymerase I</a> or <a href="/facts/RNA_polymerase_III/PthDGwwc">RNA polymerase III</a>. <a href="/facts/Pre-mRNA/4UXtKPHw">Pre-mRNA</a> undergoes a series of modifications - 5' capping, splicing and 3' <a href="/facts/Polyadenylation/ILnXUBKP">polyadenylation</a> before becoming mature mRNA that exits the <a href="/facts/Cell_nucleus/D7sFRa3c">nucleus</a> to be translated into functional proteins and capping of the 5' end is the first of these modifications. Three enzymes, <a href="/facts/RNA_triphosphatase/szsxN5Bl">RNA triphosphatase</a>, <a href="/facts/Guanylyltransferase/Frdobd4s">guanylyltransferase</a> (or CE), and <a href="/facts/Methyltransferase/MIpCDLdh">methyltransferase</a> are involved in the addition of the methylated 5' cap to the mRNA. </p>