Several 3-keto-synthases have been studied, including the soluble fatty acid synthases, those involved in polyketide synthesis, and the FAE1-like 3-ketoacyl-CoA synthases. All of these condensing enzymes have a common ancestor and an enzymatic mechanism that involves a catalytic triad consisting of Cys, His, and His/Asn. In contrast to the FAE1-like family of enzymes that mediate plant microsomal fatty acid elongation, the condensation step of elongation in animals and in fungi appears to be mediated by the Elop homologs. Curiously these proteins bear no resemblance to the well characterized 3-keto-synthases. There are three ELO genes in yeast that encode the homologous Elo1p, Elo2p, and Elo3p proteins. Elo2p and Elo3p are required for synthesis of the very long-chain fatty acids, and mutants lacking both Elo2p and Elo3p are inviable confirming that the very long-chain fatty acids are essential for cellular functions. In this study we show that heterologous expression of several Arabidopsis FAE1-like genes rescues the lethality of an elo2⌬elo3⌬ yeast mutant. We further demonstrate that FAE1 acts in conjunction with the 3-keto and trans-2,3-enoyl reductases of the elongase system. These studies indicate that even though the plantspecific FAE1 family of condensing enzymes evolved independently of the Elop family of condensing enzymes, they utilize the same reductases and presumably dehydratase that the Elop proteins rely upon.

SUMMARY ChromodomainHelicase DNA-binding protein 5 (CHD5) is a tumor suppressor mapping to 1p36—a genomic region frequently deleted in human cancer. Although CHD5 belongs to the CHD family of chromatin remodeling proteins, whether its tumor suppressive role involves an interaction with chromatin is unknown. Here we report that Chd5 binds the unmodified N-terminus of H3 through its tandem plant homeodomains (PHDs). Genome-wide ChIP studies reveal preferential binding of Chd5 to loci lacking the active mark H3K4me3, and also identify novel Chd5-targets implicated in cancer. Chd5 mutations abrogating H3 binding are unable to inhibit proliferation or to transcriptionally modulate target genes, leading to tumorigenesis in vivo. Unlike wild-type Chd5, Chd5-PHD mutants are unable to induce differentiation or to efficiently suppress growth of human neuroblastoma in vivo. Our work defines Chd5 as an N-terminally unmodified H3-binding protein and provides functional evidence that this interaction orchestrates chromatin-mediated transcriptional programs critical for tumor suppression.

The very long chain fatty acids are crucial building blocks of essential lipids, most notably the sphingolipids. These elongated fatty acids are synthesized by a system of enzymes that are organized in a complex within the endoplasmic reticulum membrane. Although several of the components of the elongase complex have recently been identified, little is known about how these proteins are organized within the membrane or about how they interact with one another during fatty acid elongation. In this study the topology of Tsc13p, the enoyl reductase of the elongase system, was investigated. The N and C termini of Tsc13p reside in the cytoplasm, and six putative membranespanning domains were identified by insertion of glycosylation and factor Xa cleavage sites at various positions. The N-terminal domain including the first membrane-spanning segment contains sufficient information for targeting to the endoplasmic reticulum membrane. Studies of the Arabidopsis Tsc13p protein revealed a similar topology. Highly conserved domains of the Tsc13p proteins that are likely to be important for enzymatic activity lie on the cytosolic face of the endoplasmic reticulum, possibly partially embedded within the membrane.Cytosolic fatty acid synthases catalyze the de novo synthesis of the 16 or 18 carbons containing fatty acids that are further elongated to very long chain fatty acids by a microsomal enzyme system, the elongase. Very long chain fatty acids are essential components of cuticular waxes and seed triacylglycerols in plants and of several classes of lipids, including the sphingolipids. Sphingolipids function in eukaryotic cells as membrane structural components, in cell interactions with surroundings, and as bioactive molecules involved in signaling and cell regulation. Sphingolipids also interact with sterols to form lipid rafts, which are involved in trafficking of plasma membrane proteins, endocytosis, and protein stability at the cell surface (1-3). The endoplasmic reticulum (ER)2 -associated elongase system is composed of four distinct enzymes that sequentially catalyze condensation between a CoA-esterified fatty-acyl substrate and malonyl-CoA, a 3-ketoacyl-CoA reduction, a 3-hydroxyacyl-CoA dehydration, and a final enoyl-CoA reduction to yield a fatty acid that is two carbon units longer than the primer (4). Several of the genes encoding components of the elongase system were first identified in Saccharomyces cerevisiae (5-9) and later in plants and mammals (10 -16). Several studies indicate that the elongase proteins are organized in a complex within the ER (5,7,17). A complete understanding of the molecular mechanism and organization of the elongase complex will require structural analysis, a challenging prospect because of the intrinsic technical difficulties associated with the purification and crystallization of membrane proteins. However, in the absence of high resolution structural data, detailed topology models aid in the design and interpretation of structure-function studies of membrane proteins. Thus, as a...

No abstract

Due to the high demand and low yield of the anti-malarial drug artemisinin in natural populations of Artemisia annua (Quinghao), an attempt has been made to enhance the artemisinin content through 4 cycles of recurrent selection (C(0)-C(3)) using selected genotypic and phenotypic traits. Based on their phenotypic and genotypic characteristics, the top 5% plants of each cycle were selected, and their seedlings were planted in poly-cross block to produce seeds for the subsequent generation. A significant increase in the artemisinin content (0.15% in C (0) to 1.16% in C (3), i.e., about 40% genetic gain over the generation) was observed. This enhancement was directly correlated with the plant height and branching intensity in all four cycles. Similarly, the PCV (phenotypic coefficient of variation) and GCV (genotypic coefficient of variation) have been observed to have a higher value for artemisinin content. The DNA marker (MAP 12) with relation to artemisinin was also identified for high yielding genotypes in all four cycles of selection. Over the four cycles of recurrent selection, the plant developed an oval appearance (Variety: CIM-Arogya) and a high artemisinin content (1.16%).