mRNA capping requires the sequential action of three enzymatic activit
ies: RNA triphosphatase, guanylyltransferase, and methyltransferase. H
ere we characterize a gene (GEL-I) believed to encode the C., elegans
capping enzyme. CEL-1 has a C-terminal domain containing motifs found
in yeast and vaccinia virus capping enzyme guanylyltransferases. The N
-terminal domain of CEL-1 has RNA tri phosphatase activity. Surprising
ly, this domain does not resemble the vaccinia virus capping enzyme bu
t does have significant sequence similarity to the protein tyrosine ph
osphatase (PTP) enzyme family. However, CEL-1 has no detectable PTP ac
tivity. The mechanism of the RNA triphosphatase is similar to that of
PTPs: the active site contains a conserved nucleophilic cysteine requi
red for activity. These results broaden the superfamily of PTP-like ph
osphatases to include enzymes with RNA substrates.