Adenosine kinase of arabidopsis. Kinetic properties and gene expression

To assess the functional significance of adenosine salvage in plants, the c DNAs and genes encoding two isoforms of adenosine kinase (ADK) were isolate d from Arabidopsis. The ADK1- and ADK2-coding sequences are very similar, s haring 92% and 89% amino acid and nucleotide identity, respectively. Each c DNA was overexpressed in Escherichia coli, and the catalytic activity of ea ch isoform was determined. Both ADKs had similar catalytic properties with a K-m and V-max/K-m for adenosine of 0.3 to 0.5 muM and 5.4 to 22 L min(-1) mg(-1) protein, respectively. The K-m and V-max/K-m for the cytokinin ribo side N-6(isopentenyl) adenosine are 3 to 5 muM and 0.021 to 0.14 L min-l mg -l protein, respectively, suggesting that adenosine is the preferred substr ate for both ADK isoforms. In Arabidopsis plants, both ADK genes are expres sed constitutively, with the highest steady-state mRNA levels being found i n stem and root. ADK1 transcript levels were generally higher than those of ADK2. ADK enzyme activity reflected relative ADK protein levels seen in im munoblots for leaves, flowers, and stems but only poorly so for roots, sili ques, and dry seeds. The catalytic properties, tissue accumulation, and exp ression levels of these ADKs suggest that they play a key metabolic role in the salvage synthesis of adenylates and methyl recycling in Arabidopsis. T hey may also contribute to cytokinin interconversion.