Methylates ribosomal protein L11 (PubMed:15317787). Preferentially recognizes free L11 before its incorporation into 50S subunits (PubMed:15317787). This function is dispensable for growth and thermostability (PubMed:15317787). L-lysyl-[protein] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl-[protein] + 3 S-adenosyl-L-homocysteine an N-terminal L-alpha-aminoacyl-[protein] + 3 S-adenosyl-L-methionine = an N-terminal trimethyl-L-alpha-aminoacyl-[protein] + 3 H(+) + 3 S-adenosyl-L-homocysteine Contains an N-terminal substrate recognition domain and a C-terminal methyltransferase domain, connected by a flexible linker. Mutant contains unmethylated protein L11 (PubMed:15317787). Null mutant is perfectly viable (PubMed:15317787). Belongs to the methyltransferase superfamily. PrmA family.