title: Structural and transcriptional analysis of plant genes encoding the bifunctional lysine ketoglutarate reductase saccharopine dehydrogenase enzyme.
abstract: BACKGROUND
  Among the dietary essential amino acids, the most severely limiting in the cereals is lysine. Since cereals make up half of the human diet, lysine limitation has quality/nutritional consequences. The breakdown of lysine is controlled mainly by the catabolic bifunctional enzyme lysine ketoglutarate reductase - saccharopine dehydrogenase (LKR/SDH). The LKR/SDH gene has been reported to produce transcripts for the bifunctional enzyme and separate monofunctional transcripts. In addition to lysine metabolism, this gene has been implicated in a number of metabolic and developmental pathways, which along with its production of multiple transcript types and complex exon/intron structure suggest an important node in plant metabolism. Understanding more about the LKR/SDH gene is thus interesting both from applied standpoint and for basic plant metabolism.
  
  
  RESULTS
  The current report describes a wheat genomic fragment containing an LKR/SDH gene and adjacent genes. The wheat LKR/SDH genomic segment was found to originate from the A-genome of wheat, and EST analysis indicates all three LKR/SDH genes in hexaploid wheat are transcriptionally active. A comparison of a set of plant LKR/SDH genes suggests regions of greater sequence conservation likely related to critical enzymatic functions and metabolic controls. Although most plants contain only a single LKR/SDH gene per genome, poplar contains at least two functional bifunctional genes in addition to a monofunctional LKR gene. Analysis of ESTs finds evidence for monofunctional LKR transcripts in switchgrass, and monofunctional SDH transcripts in wheat, Brachypodium, and poplar.
  
  
  CONCLUSIONS
  The analysis of a wheat LKR/SDH gene and comparative structural and functional analyses among available plant genes provides new information on this important gene. Both the structure of the LKR/SDH gene and the immediately adjacent genes show lineage-specific differences between monocots and dicots, and findings suggest variation in activity of LKR/SDH genes among plants. Although most plant genomes seem to contain a single conserved LKR/SDH gene per genome, poplar possesses multiple contiguous genes. A preponderance of SDH transcripts suggests the LKR region may be more rate-limiting. Only switchgrass has EST evidence for LKR monofunctional transcripts. Evidence for monofunctional SDH transcripts shows a novel intron in wheat, Brachypodium, and poplar.
authors.affiliation: Genomics and Gene Discovery Research Unit, Western Regional Research Center, USDA-ARS, 800 Buchanan Street, Albany, CA 94710, USA. olin.anderson@ars.usda.gov
authors.forename: Olin D
authors.initials: OD
authors.lastname: Anderson
authors.forename: Devin
authors.initials: D
authors.lastname: Coleman-Derr
authors.forename: Yong Q
authors.initials: YQ
authors.lastname: Gu
authors.forename: Sekou
authors.initials: S
authors.lastname: Heath
mesh_terms: D017398:Alternative Splicing; D000595:Amino Acid Sequence; D001483:Base Sequence; D022202:Chromosomes, Artificial, Bacterial; D055028:Comparative Genomic Hybridization; D018744:DNA, Plant / Q000235:genetics; D020224:Expressed Sequence Tags; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D018745:Genome, Plant; D007438:Introns; D008239:Lysine / Q000378:metabolism*; D008969:Molecular Sequence Data; D010802:Phylogeny; D032107:Populus / Q000201:enzymology / Q000235:genetics; D022821:RNA Splice Sites; D012443:Saccharopine Dehydrogenases / Q000235:genetics*; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D014908:Triticum / Q000201:enzymology / Q000235:genetics*
chemical_list: D018744:DNA, Plant; D022821:RNA Splice Sites; D012443:Saccharopine Dehydrogenases; D008239:Lysine