tRNA isopentenyltransferase from Zea mays L. Characterization of the isopentenylation reaction of tRNA, oligo (A) and other nucleic acids.

The extraction and purification of the tRNA isopentenyltransferase from maize root tips and kernels are reported. The relative amounts of this enzyme in different organs of maize have been determined. Root tips have the highest enzyme activities, followed by kernels and young leaves. Old leaves exhibit very low activity. The molecular mass of the monomeric enzyme was determined to be 57000 - 63000 dalton. pH and Mg2 optima are in full agreement with the data reported for the enzymes from yeast and Escherichia coli. Spermine enhances the isopentenylation of tRNA from kernels. The "Km" values of KMnO4-treated or untreated bulk tRNA from yeast and maize root tips and kernels were in the range of 10 to 34 micrometer while the Km values of of single species like tRNASer4 from rat liver and tRNATyrKMnO4 from E. coli were 1.1 and 6.6 micrometer, respectively. The tRNA isopentenyltransferase from maize root tips and kernels catalyzes the incorporation of 2-isopentenyl groups into (Ap)3-7A, endogenous bulk oligonucleotos from maize root tips and kernels, and into poly(A), RNA from MS-2 phages and, to a very low extent, into adenosine. The Km values of (Ap)3-7 A varied in the range of 250 to 750micrometer. Although oligonucleotides have less affinity to the enzyme, the formation of i6A within oligonucleotides may occur in vivo, due to their higher concentrations in cells.