No abstract

A novel plant-specific type III polyketide synthase (PKS) that catalyzes formation of a pentaketide chromone, 5,7-dihydroxy-2-methylchromone, from five molecules of malonyl-CoA, was cloned and sequenced from aloe (Aloe arborescens). Site-directed mutagenesis revealed that Met207 (corresponding to Thr197 in CHS) determines the polyketide chain length and the product specificity of the enzyme; remarkably, replacement of a single amino acid residue, Met207, with Gly yielded a mutant enzyme that efficiently produces aromatic octaketides, SEK4 and SEK4b, the products of the minimal PKS for actinorhodin (act from Streptomyces coelicolor), from eight molecules of malonyl-CoA. This provided new insights into the catalytic functions and specificities of the CHS-superfamily type III PKS enzymes.

Bacterial translation elongation factor G (EF-G) catalyzes translocation during peptide elongation and mediates ribosomal disassembly during ribosome recycling in concert with the ribosomal recycling factor (RRF). Two homologs of EF-G have been identified in mitochondria from yeast to man, EF-G1mt and EF-G2mt. Here, we demonstrate that the dual function of bacterial EF-G is divided between EF-G1mt and EF-G2mt in human mitochondria (RRFmt). EF-G1mt specifically catalyzes translocation, whereas EF-G2mt mediates ribosome recycling with human mitochondrial RRF but lacks translocation activity. Domain swapping experiments suggest that the functional specificity for EF-G2mt resides in domains III and IV. Furthermore, GTP hydrolysis by EF-G2mt is not necessary for ribosomal splitting, in contrast to the bacterial-recycling mode. Because EF-G2mt represents a class of translational GTPase that is involved in ribosome recycling, we propose to rename this factor mitochondrial ribosome recycling factor 2 (RRF2mt).

Normal human fibroblasts (MRC‐5 or NTI‐4) were transfected with pSV2‐neo plasmid DNA. Fifty G418‐resistant fibroblast clones were isolated and independently fused to mouse A9 cells. The cell hybrids were selected and isolated in the medium containing G418 plus ouabaln. Since micronuclei were more efficiently induced in these hybrids compared to parental human fibroblasts by colcemid treatment, the transfer of neo‐tagged human chromosomes in the hybrids to mouse A9 cells was performed via microcell fusion. Two hundred A9 microcell hybrids were isolated and karyotyped. Among them, thirteen microcell clones, each containing a single human chromosome 1, 2, 5, 6, 7, 8, 10,11,12,15,18,19 or 20 were established. Isozyme analyses confirmed the presence of each human chromosome in these A9 microcell clones. The results of Southern blot and chromosomal in situ hybridization analyses indicate that the human chromosomes in these clones were tagged with pSV2‐ neo plasmid DNA.

Benzalacetone synthase (BSA) is a novel plant‐specific polyketide synthase that catalyzes a one step decarboxylative condensation of 4‐coumaroyl‐CoA with malonyl‐CoA to produce the C6–C4 skeleton of phenylbutanoids in higher plants. A cDNA encoding BAS was for the first time cloned and sequenced from rhubarb (Rheum palmatum), a medicinal plant rich in phenylbutanoids including pharmaceutically important phenylbutanone glucoside, lindleyin. The cDNA encoded a 42‐kDa protein that shares 60–75% amino‐acid sequence identity with other members of the CHS‐superfamily enzymes. Interestingly, R. palmatum BAS lacks the active‐site Phe215 residue (numbering in CHS) which has been proposed to help orient substrates and intermediates during the sequential condensation of 4‐coumaroyl‐CoA with malonyl‐CoA in CHS. On the other hand, the catalytic cysteine‐histidine dyad (Cys164–His303) in CHS is well conserved in BAS. A recombinant enzyme expressed in Escherichia coli efficiently afforded benzalacetone as a single product from 4‐coumaroyl‐CoA and malonyl‐CoA. Further, in contrast with CHS that showed broad substrate specificity toward aliphatic CoA esters, BAS did not accept hexanoyl‐CoA, isobutyryl‐CoA, isovaleryl‐CoA, and acetyl‐CoA as a substrate. Finally, besides the phenylbutanones in rhubarb, BAS has been proposed to play a crucial role for the construction of the C6–C4 moiety of a variety of natural products such as medicinally important gingerols in ginger plant.

The crystal structures of a wild-type and a mutant PCS, a novel plant type III polyketide synthase from a medicinal plant, Aloe arborescens, were solved at 1.6 A resolution. The crystal structures revealed that the pentaketide-producing wild-type and the octaketide-producing M207G mutant shared almost the same overall folding, and that the large-to-small substitution dramatically increases the volume of the polyketide-elongation tunnel by opening a gate to two hidden pockets behind the active site of the enzyme. The chemically inert active site residue 207 thus controls the number of condensations of malonyl-CoA, solely depending on the steric bulk of the side chain. These findings not only provided insight into the polyketide formation reaction, but they also suggested strategies for the engineered biosynthesis of polyketides.

We examined the ability of human chromosome 11 derived from normal fibroblast cells to suppress the tumorigenicity of SiHa cells, a human cervical tumor cell line. Using DNA transfection, the human chromosome was tagged with a selectable marker (the pSV2neo gene, which encodes resistance to the antibiotic G418), transferred to mouse A9 cells by cell hybridization and microcell transfer techniques, and then transferred to SiHa cells by microcell transfer. These procedures resulted in the appearance of 15 independent, G418-resistant clones, 5 of which had one or two extra copies of an intact human chromosome 11. In situ chromosomal hybridization of these clones with the pSV2neo plasmid revealed the presence of a neo-tagged human chromosome 11 in all of the five SiHa-microcell hybrids. Two SiHa-microcell hybrids that contained a single copy of neo-tagged human chromosome 12 were also isolated by the same methods. The tumorigenicities of SiHa clones with one or two extra copies of chromosome 11 (SiHa-11) were suppressed; four of the five SiHa-11 clones formed no tumors in nude mice, whereas both parental SiHa cells and SiHa cells with an extra chromosome 12 formed tumors within 30 d. One SiHa-11 cell clone formed a single tumor 90 d after injection. This rare tumor had lost one copy of chromosome 11 and rapidly formed tumors when reinjected. These results indicate that the introduction of a single copy of normal human chromosome 11, but not chromosome 12, suppresses the tumorigenicity of SiHa cells, indicating the presence on human chromosome 11 of a putative tumor-suppressor gene (or genes) for human cervical tumors.

Curcuminoid synthase (CUS) from Oryza sativa is a plant-specific type III polyketide synthase (PKS) that catalyzes the remarkable one-pot formation of the C 6 -C 7 -C 6 diarylheptanoid scaffold of bisdemethoxycurcumin, by the condensation of two molecules of 4-coumaroyl-CoA and one molecule of malonyl-CoA. The crystal structure of O. sativa CUS was solved at 2.5-Å resolution, which revealed a unique, downward expanding active-site architecture, previously unidentified in the known type III PKSs. The large active-site cavity is long enough to accommodate the two C 6 -C 3 coumaroyl units and one malonyl unit. Furthermore, the crystal structure indicated the presence of a putative nucleophilic water molecule, which forms hydrogen bond networks with Ser351-Asn142-H 2 O-Tyr207-Glu202, neighboring the catalytic Cys174 at the active-site center. These observations suggest that CUS employs unique catalytic machinery for the one-pot formation of the C 6 -C 7 -C 6 scaffold. Thus, CUS utilizes the nucleophilic water to terminate the initial polyketide chain elongation at the diketide stage. Thioester bond cleavage of the enzyme-bound intermediate generates 4-coumaroyldiketide acid, which is then kept within the downward expanding pocket for subsequent decarboxylative condensation with the second 4-coumaroyl-CoA starter, to produce bisdemethoxycurcumin. The structure-based site-directed mutants, M265L and G274F, altered the substrate and product specificities to accept 4-hydroxyphenylpropionyl-CoA as the starter to produce tetrahydrobisdemethoxycurcumin. These findings not only provide a structural basis for the catalytic machinery of CUS but also suggest further strategies toward expanding the biosynthetic repertoire of the type III PKS enzymes.