The TbMTr1 spliced leader RNA cap 1 2′-O-ribose methyltransferase from Trypanosoma brucei acts with substrate specificity

Abstract

In metazoa cap 1 (m7GpppNmp-RNA) is linked to higher levels of translation; however, the enzyme responsible remains unidentified. We have validated the first eukaryotic encoded cap 1 2′-O-ribose methyltransferase, TbMTr1, a member of a conserved family that modifies the first transcribed nucleotide of spliced leader and U1 small nuclear RNAs in the kinetoplastid protozoan Trypanosoma brucei. In addition to cap 0 (m 7GpppNp-RNA), mRNA in these parasites has ribose methylations on the first four nucleotides with base methylations on the first and fourth (m 7 Gpppm6,6AmpAmpCmpm3Ump-SLRNA) conveyedviatranssplicing of a universal spliced leader. The function of this cap 4 is unclear. Spliced leader is the majority RNA polymerase II transcript; the RNA polymerase III-transcribed U1 small nuclear RNA has the same first four nucleotides as spliced leader, but it receives an m2,2,7G cap with hypermethylation at position one only (m2,2,7Gpppm 6,6AmpApCpUp-U1 snRNA). Here we examine the biochemical properties of recombinant TbMTr1. Active over a pH range of 6.0 to 9.5, TbMTr1 is sensitive to Mg2+. Positions Lys95-Asp204-Lys 259-Glu285 constitute the conserved catalytic core. A guanosine cap on RNA independent of its N7 methylation status is required for substrate recognition, but anm2,2,7G-cap is not recognized. TbMTr1 favors the spliced leader 5′ sequence, as reflected by a preference for A at position 1 and modulation of activity for substrates with base changes at positions 2 and 3. With similarities to human cap 1 methyltransferase activity, TbMTr1 is an excellent model for higher eukaryotic cap 1 methyltransferases and the consequences of cap 1 modification. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.