An active ribonucleotide reductase from Arabidopsis thaliana -: Cloning, expression and characterization of the large subunit

In all living organisms, deoxyribonucleotides, the DNA precursors, are produced by reduction of the corresponding ribonucleotides catalyzed by ribonucleotide reductase. In mammals as in Escherichia coli, the enzyme consists of two proteins. Protein R1 is the props reductase as it contains, in the substrate binding site, the reducing active cysteine pair. Protein R2 provides a catalytically essential organic radical. Here we report the cloning, expression, purification and characterization of protein R1 from Arabidopsis thaliana. Expression in E. coli was made possible by coexpression of tRNA(Arg4) which is required for the utilization of AGA and AGG as codons for arginines. Protein R1 shows extensive similarities with protein R1 from mammals: (a) it shows 69% amino-acid sequence identity to human and mouse R1 protein; (b) it is active during CDP reduction by dithiothreitol, in the presence of protein R2 (c) activity is stimulated by thioredoxin and ATP and is inhibited by dATP, showing that as in the mammalian enzyme, the plant ribonucleotide reductase seems to be allosterically regulated by positive (ATP) and negative (dATP) effecters.