Plant Physiol, September 2002, Vol. 130, pp. 147-154
The Bifunctional LKR/SDH Locus of Plants
Also Encodes a Highly Active Monofunctional Lysine-Ketoglutarate
Reductase Using a Polyadenylation Signal Located within an
Intron1,[w]
Guiliang
Tang,2
Xiaohong
Zhu,2
Bertrand
Gakiere,
Hanna
Levanony,
Anat
Kahana, and
Gad
Galili*
Department of Plant Sciences, The Weizmann Institute of Science,
Rehovot 76100 Israel
Both plants and animals catabolize lysine (Lys) via
two consecutive enzymes, Lys-ketoglutarate reductase (LKR) and
saccharopine dehydrogenase (SDH), which are linked on a single
polypeptide encoded by a single LKR/SDH
gene. We have previously shown that the Arabidopsis
LKR/SDH gene also encodes a
monofunctional SDH that is transcribed from an internal promoter. In
the present report, we have identified two cDNAs derived from cotton
(Gossypium hirsutum) boll abscission zone that encode a
novel enzymatic form of Lys catabolism, i.e. a catabolic monofunctional
LKR. The monofunctional LKR mRNA is also encoded by the
LKR/SDH gene, using two weak
polyadenylation sites located within an intron. In situ mRNA
hybridization and quantitative reverse transcriptase-polymerase chain
reaction analyses also suggest that the cotton monofunctional LKR is
relatively abundantly expressed in parenchyma cells of the abscission
zone. DNA sequence analysis of the
LKR/SDH genes of Arabidopsis, maize (Zea mays), and tomato (Lycopersicon
esculentum) suggests that these genes can also encode a
monofunctional LKR mRNA by a similar mechanism. To test whether the
LKR/SDH and monofunctional LKR enzymes possess different biochemical
properties, we used recombinant Arabidopsis LKR/SDH and monofunctional
LKR enzymes expressed in yeast (Saccharomyces
cerevisiae) cells. The Km of the
monofunctional LKR to Lys was nearly 10-fold lower than its counterpart
that is linked to SDH. Taken together, our results suggest that the LKR/SDH locus of plants is a
super-composite locus that can encode three related but distinct
enzymes of Lys catabolism. These three enzymes apparently operate in
concert to finely regulate Lys catabolism during plant development.
1
This work was supported by grants from the
FrameWork Program of the Commission of the European Communities and the
Israel Academy of Sciences and Humanities, National Council for
Research and Development. G.T. was supported in part by a Leon and
Kathe Fallek scholarship. G.G. is an incumbent of the Bronfman Chair of
Plant Sciences.
2
These authors contributed equally to the paper.
[w]
The online version of this article contains Web-only
data. The supplemental material is available at
www.plantphysiol.org.
*
Corresponding author; e-mail gad.galili{at}weizmann.ac.il; fax
972-8-9344181.
© 2002 American Society of Plant Physiologists
