THE DIM1 GENE RESPONSIBLE FOR THE CONSERVED M(2)(6)AM(2)(6)A DIMETHYLATION IN THE 3'-TERMINAL LOOP OF 18-S RIBOSOMAL-RNA IS ESSENTIAL IN YEAST

Biogenesis of cytoplasmic ribosomes universally involves methylation of ribosomal RNA. Little genetic evidence is available about the functional role(s) of this conserved posttranscriptional modification. The only known methylase gene involved in rRNA maturation is ksgA in Escherichia coli, which directs dimethylation of two adjacent; adenosines (m(2)(6)A(1518)m(2)(6)A(1519)) in the loop of a conserved hairpin near the 3'-end of 16 S rRNA. This . tandem methylation is the only rRNA modification common to pre and eukaryotes. Disruption of ksgA confers resistance to the aminoglycoside antibiotic kasugamycin without significantly impairing viability. Here we report the cloning of the DIM1 gene encoding the homolog 18 S rRSA dimethylase in Saccharomyces cerevisiae. The yeast enzyme is evolutionary related to the ksgA protein. It carries a distinctive lysine-rich-N-terminal extension with a potential protein kinase C phosphorylation site. Like ksgA, DIM1 belongs to the erm family of prokaryotic 23 S rRNA dimethylases responsible for erythromycin resistance. Surprisingly, disruption of DIM1 turns out to be lethal in yeast.