Conversion of a DNA ligase into an RNA capping enzyme

In eukaryotes, newly synthesised mRNA is 'capped' by the addition of GMP to the 5' end by RNA capping enzymes. Recent structural studies have shown th at RNA capping enzymes and DNA ligases have similar protein folds, suggesti ng a conserved catalytic mechanism. To explore these similarities we have p roduced a chimeric enzyme comprising the N-terminal domain 1 of a DNA ligas e fused to the C-terminal domain 2 of a mRNA capping enzyme. This report sh ows that this hybrid enzyme retains adenylation activity, characteristic of DNA ligases but, remarkably, the chimera has ATP-dependent mRNA capping ac tivity. This is the first observation of ATP-dependent RNA capping. These r esults suggest that nucleotidyltransferases may have evolved from a common ancestral gene.