Characterization of S -Adenosylmethionine: Ribosomal Ribonucleic Acid-Adenine (N 6 -) Methyltransferase of Escherichia coli Strain B

Abstract

This study is concerned with the isolation and characterization of the enzyme, S -adenosylmethionine:ribosomal ribonucleic acid-adenine (N 6− ) methyl-transferase [rRNA-adenine (N 6 -) methylase] of Escherichia coli strain B, which is responsible for the formation of N 6 -methyladenine moieties in ribosomal ribonucleic acids (rRNA). A 1,500-fold purified preparation of the species-specific methyltransferase methylates a limited number of adenine moieties in heterologous rRNA ( Micrococcus lysodeikticus and Bacillus subtilis ) and methyl-deficient homologous rRNA. The site recognition mechanism does not require intact 16 or 23 S rRNA. The enzyme does not utilize transfer ribonucleic acid as a methyl acceptor nor does it synthesize 2-methyladenine or N 6 -dimethyladenine moieties. Mg 2+ , spermine, K + , and Na + increase the reaction rate but not the extent of methylation; elevated concentrations of the cations inhibit markedly. The purified preparations utilize 9-β-ribosyl-2,6-diaminopurine (DAPR) as a methyl acceptor with the synthesis of 9-β-ribosyl-6-amino-2-methylaminopurine. A comparison of the two activities demonstrated that one methyltransferase is responsible for the methylation of both DAPR and rRNA. This property provides a sensitive assay procedure unaffected by ribonucleases and independent of any specificity exhibited by rRNA methyl acceptors.