EXPRESSION AND CHARACTERIZATION OF AN RNA CAPPING ENZYME ENCODED BY CHLORELLA VIRUS PBCV-1

We report that the A103R protein of Chlorella virus PBCV-1 is an mRNA capping enzyme that catalyzes the transfer of GRIP from GTP to the 5' diphosphate end of RNA. This is a two-step reaction in which the enzym e first condenses with GTP to form a covalent enzyme-GMP intermediate and then transfers the GMP to an RNA acceptor to form a GpppN cap. Pur ified recombinant A103R is a 38-kDa monomer that larks RNA. (guanine-7 -) methyltransferase activity. With respect to its size, amino acid se quence, and biochemical properties, A103R is more closely related to t he yeast RNA guanylyltransferases than it is to the multifunctional ca pping enzymes coded for by other large DNA viruses - the poxviruses an d African swine fever virus, We surmise that in order to cap its trans cripts, PBCV-1 must either encode additional 5' processing activities or else rely on the host alga to provide these functions.