The bifunctional LKR/SDH locus of plants also encodes a highly active monofunctional lysine-ketoglutarate reductase using a polyadenylation signal located within an intron.
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 K(m) 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.
Tang
Guiliang
G
Department of Plant Sciences, The Weizmann Institute of Science, Rehovot 76100 Israel.
Zhu
Xiaohong
X
Gakiere
Bertrand
B
Levanony
Hanna
H
Kahana
Anat
A
Galili
Gad
G
D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics; D001483:Base Sequence; D003368:Gossypium / Q000201:enzymology* / Q000235:genetics; D017403:In Situ Hybridization; D007438:Introns / Q000235:genetics*; D018551:Solanum lycopersicum / Q000201:enzymology / Q000235:genetics; D008239:Lysine / Q000378:metabolism; D008969:Molecular Sequence Data; D026723:Polyadenylation / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D012443:Saccharopine Dehydrogenases / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid; D015398:Signal Transduction / Q000235:genetics; D003313:Zea mays / Q000201:enzymology / Q000235:genetics
D012333:RNA, Messenger; D012443:Saccharopine Dehydrogenases; D008239:Lysine