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Methylation of ribosomal proteins in bacteria: evidence of conserved modification of the eubacterial 50S subunit

Methylation of the 50S ribosomal proteins from Bacillus stearothermophilus, Bacillus subtilis, Alteromonas espejiana, and Halobacterium cutirubrum was measured after the cells were grown in the presence of [1-14C]methionine or [methyl-3H]methionine or both. Two-dimensional polyacrylamide gel electrophoretic analysis revealed, in general, similar relative electrophoretic mobilities of the methylated proteins from each eubacterium studied. Proteins known to be structurally and functionally homologous in several microorganisms were all methylated. Thus, the following group of proteins, which appear to be involved in peptidyltransferase or in polyphenylalanine-synthesizing activity in B. stearothermophilus (P.E. Auron and S. R. Fahnestock, J. Biol. Chem. 256:10105-10110, 1981), were methylated (possible Escherichia coli methylated homologs are indicated in parentheses): BTL5(EL5), BTL6(EL3), BTL8(EL10), BTL11(EL11), BTL13(EL7L12) and BTL20b(EL16). In addition, the pentameric ribosomal complex BTL13 X BTL8, analogous to the complex EL7L12 X EL10 of E. coli, contained methylated proteins. Analysis of the methylated amino acids in the most heavily methylated proteins, BSL11 from B. subtilis and BTL11 from B. stearothermophilus, showed the presence of epsilon-N-trimethyllysine as the major methylated amino acid in both proteins, in agreement with known data for E. coli. In addition, BSL11 appeared to contain trimethylalanine, a characteristic, modified amino acid previously described only in EL11 from E. coli. These results and those previously obtained from other bacteria indicate a high degree of conservation for ribosomal protein methylation and suggest an important, albeit unknown, role for the modification of these components in eubacterial ribosomes.