Identification of a Novel Methyltransferase, Bmt2, Responsible for the N-1-methyl-adenosine Base Modification of 25S RRNA in Saccharomyces Cerevisiae

The 25S rRNA of yeast contains several base modifications in the functionally important regions. The enzymes responsible for most of these base modifications remained unknown. Recently, we identified Rrp8 as a methyltransferase involved in m(1)A645 modification of 25S rRNA. Here, we discovered a previously uncharacterized gene YBR141C to be responsible for second m(1)A2142 modification of helix 65 of 25S rRNA. The gene was identified by reversed phase-HPLC screening of all deletion mutants of putative RNA methyltransferase and was confirmed by gene complementation and phenotypic characterization. Because of the function of its encoded protein, YBR141C was named BMT2 (base methyltransferase of 25S RNA). Helix 65 belongs to domain IV, which accounts for most of the intersubunit surface of the large subunit. The 3D structure prediction of Bmt2 supported it to be an Ado Met methyltransferase belonging to Rossmann fold superfamily. In addition, we demonstrated that the substitution of G180R in the S-adenosyl-L-methionine-binding motif drastically reduces the catalytic function of the protein in vivo. Furthermore, we analysed the significance of m(1)A2142 modification in ribosome synthesis and translation. Intriguingly, the loss of m(1)A2142 modification confers anisomycin and peroxide sensitivity to the cells. Our results underline the importance of RNA modifications in cellular physiology.