Escherichia coli dimethylallyl diphosphate:tRNA dimethylallyltransferase:: Site-directed mutagenesis of highly conserved residues

Dimethylallyl diphosphate:tRNA dimethylallyltransferase (DMAPP-tRNA transferase) catalyzes alkylation of the exocyclic amine of adenosine at position 37 in some tRNAs by the hydrocarbon moiety of dimethylallyl diphosphate (DMAPP). A multiple-sequence alignment of 28 gene sequences encoding DMAPP-tRNA transferases from various organisms revealed considerable homology, including II charged, 12 polar, and four aromatic amino acids that are highly conserved or conservatively substituted. Site-directed mutants were constructed for all of these amino acids, and a tripeptide Glu-Glu-Phe alpha -tubulin epitope was appended to the C-terminus of the protein to facilitate separation by immunoaffinity chromatography of overproduced mutant enzymes from coexpressed chromosomally encoded wild-type DMAPP-tRNA transferase. Steady-state kinetic constants were measured for wild-type DMAPP-tRNA transferase and the site-directed mutants using DMAPP and a 17-base RNA oligoribonucleotide corresponding to the stem-loop region of tRNA(Phe) as substrates. Substantial changes in k(cat), K-m(DMAPP), and/or K-m(RNA) were seen for several of the mutants, suggesting possible roles for these residues in substrate binding and catalysis.