   2.1.1.171 | S-adenosyl-L-methionine + guanine966 in 16S rRNA |
the enzyme uses unmethylated 30S subunits as a substrate, but not free unmethylated 16S rRNA. Binding of ribosomal proteins S7, S9, and S19 to unmodified 16S rRNA individually and in all possible combinations shows that S7 plus S19 are sufficient to block methylation by the m5C967 methyltransferase, while simultaneously inducing methylation by the m2G966 methyltransferase. A purified complex containing stoichiometric amounts of proteins S7, S9, and S19 bound to 16S rRNA is isolated and shown to possess the same methylation properties as 30S subunits, that is, the ability to be methylated by the m2G966 methyltransferase but not by the m5C967 methyltransferase. Since binding of S19 requires prior binding of S7, which has no effect on methylation when bound alone, the switch in methylase specificity is attributed solely to the presence of RNA-bound S19. Single-omission reconstitution of 30S subunits deficient in S19 results in particles that could not be efficiently methylated by either enzyme. Thus while binding of S19 is both necessary and sufficient to convert 16S rRNA into a substrate of the m2G966 methyltransferase, binding of either S19 alone or some other protein or combination of proteins to the 16S rRNA can abolish activity of the m5C967 methyltransferase. Binding of S19 to 16S rRNA is known to cause local conformational changes in the 960-975 stem-loop structure surrounding the two methylated nucleotides |
