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The plant mitochondrial genome is retained in a multipartite structure that arises by a process of repeat-mediated homologous recombination. Low-frequency ectopic recombination also occurs, often producing sequence chimeras, aberrant ORFs, and novel subgenomic DNA molecules. This genomic plasticity may distinguish the plant mitochondrion from mammalian and fungal types. In plants, relative copy number of recombination-derived subgenomic DNA molecules within mitochondria is controlled by nuclear genes, and a genomic shifting process can result in their differential copy number suppression to nearly undetectable levels. We have cloned a nuclear gene that regulates mitochondrial substoichiometric shifting in Arabidopsis. The CHM gene was shown to encode a protein related to the MutS protein of Escherichia coli that is involved in mismatch repair and DNA recombination. We postulate that the process of substoichiometric shifting in plants may be a consequence of ectopic recombination suppression or replication stalling at ectopic recombination sites to effect molecule-specific copy number modulation.

This review focuses on the monoterpene, sesquiterpene, and diterpene synthases of plant origin that use the corresponding C10, C15, and C20 prenyl diphosphates as substrates to generate the enormous diversity of carbon skeletons characteristic of the terpenoid family of natural products. A description of the enzymology and mechanism of terpenoid cyclization is followed by a discussion of molecular cloning and heterologous expression of terpenoid synthases. Sequence relatedness and phylogenetic reconstruction, based on 33 members of the Tps gene family, are delineated, and comparison of important structural features of these enzymes is provided. The review concludes with an overview of the organization and regulation of terpenoid metabolism, and of the biotechnological applications of terpenoid synthase genes.

Photosynthetic eukaryotes typically possess two distinct glyceraldehyde-3-phosphate dehydrogenases, an NAD+ -specific enzyme in the cytosol (GapC: EC 1.2.1.12) and an NADP+ -dependent enzyme in the chloroplast (GapAB: EC 1.2.1.13). The gymnosperm Pinus sylvestris is an exception in that it is known to express a gene encoding a transit peptide-bearing GapC-like subunit that is imported into chloroplasts (GapCp), but the enzymatic properties of this novel GAPDH have not been described from any source. We have expressed the mature GapCp unit from Pinus in Escherichia coli and have characterized the active enzyme. GapCp has a specific activity of 89 units per milligram and is strictly NAD+ -dependent, showing no detectable activity with NADP+. Values of the apparent Km for NAD+ and glyceraldehyde-3-phosphate were determined as 62 and 344 microM, respectively. The Pinus GapCpl gene possesses 12 introns, two in the region encoding the transit peptide and ten in the region encoding the mature subunit, all of which are found at positions strictly conserved across genes for higher plant GapC. A cDNA encoding a homologue of GapCp was isolated from the heterosporous fern Marsilea quadrifolia, indicating that NAD+ -dependent chloroplast GAPDH also occurs in other higher plants.

Angiosperms and algae possess two distinct glyceraldehyde-3-phosphate dehydrogenase (GAPDH) enzymes, an NAD(+)-dependent tetramer involved in cytosolic glycolysis and an NADP(+)-dependent enzyme of the Calvin cycle in chloroplasts. We have found that the gymnosperm Pinus sylvestris possesses, in addition to these, a nuclear-encoded, plastid-specific, NAD(+)-dependent GAPDH, designated GapCp, which has not previously been described from any plant. Several independent full-size cDNAs for this enzyme were isolated which encode a functional transit peptide and mature subunit very similar to that of cytosolic GAPDH of angiosperms and algae. A molecular phylogeny reveals that chloroplast GapCp and cytosolic GapC arose through gene duplication early in chlorophyte evolution. The GapCp gene is expressed as highly as that for GapC in light-grown pine seedlings. These findings suggest that aspects of compartmentalized sugar phosphate metabolism may differ in angiosperms and gymnosperms and furthermore underscore the contributions of endosymbiotic gene transfer and gene duplication to the nuclear complement of genes for enzymes of plant primary metabolism.