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Plant Physiol, December 2000, Vol. 124, pp. 1775-1785
Adenosine Kinase of Arabidopsis. Kinetic Properties and Gene
Expression1
Barbara A.
Moffatt,*
Li
Wang,2
Mike S.
Allen,
Yvonne Y.
Stevens,3
Wensheng
Qin,4
Jamie
Snider, and
Klaus
von Schwartzenberg
Department of Biology, University of Waterloo, Waterloo, Ontario,
Canada N2L 3G1 (B.M., L.W., M.A., Y.S., W.Q., J.S.); and Institut
für Angewandte Botanik, D-20355 Hamburg, Germany
(K.v.S.)
To assess the functional significance of adenosine salvage in
plants, the cDNAs and genes encoding two isoforms of adenosine kinase
(ADK) were isolated from Arabidopsis. The
ADK1- and ADK2-coding sequences are very
similar, sharing 92% and 89% amino acid and nucleotide identity,
respectively. Each cDNA was overexpressed in Escherichia
coli, and the catalytic activity of each isoform was
determined. Both ADKs had similar catalytic properties with a
Km and
Vmax/Km for
adenosine of 0.3 to 0.5 µM and 5.4 to 22 L
min 1 mg1 protein, respectively. The
Km and
Vmax/Km for the
cytokinin riboside N6(isopentenyl) adenosine are 3 to 5 µM and 0.021 to 0.14 L min1
mg1 protein, respectively, suggesting that adenosine is
the preferred substrate for both ADK isoforms. In Arabidopsis
plants, both ADK genes are expressed constitutively,
with the highest steady-state mRNA levels being found in stem and root.
ADK1 transcript levels were generally higher than those
of ADK2. ADK enzyme activity reflected relative ADK
protein levels seen in immunoblots for leaves, flowers, and stems but
only poorly so for roots, siliques, and dry seeds. The catalytic
properties, tissue accumulation, and expression levels of these ADKs
suggest that they play a key metabolic role in the salvage synthesis of
adenylates and methyl recycling in Arabidopsis. They may also
contribute to cytokinin interconversion.
1
This work was supported by a grant from the
Natural Science and Engineering Research Council (to B.M.) and by a
grant from Deutsche Forschungsgemeinschaft (to K.v.S.).
2
Present address: Eastern Cereal and Oilseed Research
Centre, Agriculture and Agri-Food Canada, 960 Carling Avenue, K.W.
Neatby Building, Room 4017, Ottawa, ON, Canada K1A 0C6.
3
Present address: Regional Primate Research Centre,
University of Washington, Box 357330, Seattle, WA.
4
Present address: Department of Medicine, 1017 Etherington Hall, Queen's University, Kingston, ON, Canada K7L 3N6.
*
Corresponding author; e-mail moffatt{at}sciborg.uwaterloo.ca; fax
519-888-4567.
© 2000 American Society of Plant Physiologists
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