Comparative analysis of the predicted amino acid sequences of a number of plant O-methyltransferase cDNA clones show that they share some 32-71% sequence identity, and can be grouped according to the different compounds they utilise as substrates. Five highly conserved regions are proposed as a signature for plant O-methyltransferases, two of which (regions I and IV) are believed to be involved in S-adenosyl-L-methionine and metal binding, respectively. The glycine-rich signature regions include a 36 amino acid domain which is located in the mid-terminal section of the carboxy terminus of most O-methyltransferase sequences. Cladistic analysis of the amino acid sequences suggests that plant O-methyltransferases may have arisen from common ancestral genes that were driven by different structural and/or functional requirements, and whose descendants segregated into different biochemical species. A comprehensive classification of plant O-methyltransferases is proposed following the guidelines of the Commission of Plant Gene Nomenclature.

Abstract. The uptake of [1 -14C]isopentenyl diphosphate by intact plastids purified from cell suspensions of Vitis vin~era L. cv. Muscat de Frontignan was investigated using vacuum-filtration and silicone-oil-filtering techniques. Transport across the plastid envelope which was stimulated by cations, such as Mg 2+ and Mn 2+, was characterized by a K m of approx. 0.5 mM and a Vm, X of 25 nmol.(mg protein)-~.h -l. The data showed that isopentenyl diphosphate apparently accumulated in the plastid against a concentration gradient. The involvement of a protein carrier was suggested by the strong inhibition of the uptake by compounds which are known to block SH groups. Thus, the saturation kinetics together with the pH optimum (7.5-8), the temperature dependence (maximum incorporation at 37 ~ and the competitive inhibition by a structural analogue of the substrate (aminophenylethyl diphosphate) provided evidence for a mechanism of uptake by facilitated diffusion. The carrier identified may thus play a major role in supplying the plastid compartment with isopentenyI diphosphate for isoprenoid biosynthesis.

An abundant 17 kDa protein which was isolated and characterized from 10-day old healthy root tissue of white lupin (Lupinus albus) proved to have a high sequence similarity to pathogenesis-related proteins found in other species. Subsequently, a corresponding clone (LaPR-10) was identified in a cDNA library prepared from the same tissue that exhibited a high amino acid sequence similarity to a number of the PR-10 family proteins. The clone contains an open reading frame encoding a polypeptide of 158 amino acids, with a predicted molecular mass of 16,905 Da and an isoelectric point of 4.66. Southern blot analysis indicates that LaPR-10 is likely a single-copy gene, or a member of a small gene family. The clone was expressed in Escherichia coli, and its protein product was purified to near homogeneity. Both the native and the recombinant proteins were immunorecognized by antibodies raised against pea PR-10 proteins, and exhibited a ribonucleolytic activity against several RNA preparations, including lupin root total RNA. Characterization of its enzymatic properties indicates that the LaPR-10 protein belongs to the class II ribonucleases. We present evidence that the white lupin 17 kDa protein is constitutively expressed during all stages of root development and, to a lesser extent, in other plant parts. In addition, we demonstrate the presence, in the LaPR-10 amino acid sequence, of a number of motifs that are common to most PR-10 proteins, as well as a RGD motif that is shared only with the alfalfa SRG1 sequence.

A geranyl diphosphate synthase (EC 2.5.1.1), which catalyzes the formation of geranyl diphosphate from dimethylallyl diphosphate and isopentenyl diphosphate, was isolated from Vitis vinifera 1. cv Muscat de Frontignan cell cultures. Purification of the enzyme was achieved successively by ammonium sulfate precipitation and chromatography on DEAE-Sephacel, hydroxylapatite, Mono Q, Phenyl Superose, Superose 12, and preparative nondenaturing polyacrylamide gels. l h e enzyme formed only geranyl diphosphate as a product. In all cases, neither neryl diphosphate, the cis isomer, nor farnesyl diphosphate was detected. l h e enzyme showed a native molecular mass of 68 f 5 kD as determined by gel permeation.O n sodium dodecyl sulfate polyacrylamide gels, geranyl diphosphate synthase purified to electrophoretic homogeneity migrated with a molecular mass of 66 f 2 kD. Michaelis constants for isopentenyl diphosphate and dimethylallyl diphosphate were 8.5 and 56.8 p~, respectively. The enzyme required MnZ+ and Mgz+ as cofactors and its activity was enhanced by Triton X-100. lnorganic pyrophosphate, aminophenylethyl diphosphate, and geranyl diphosphate had inhibitory effects on the enzyme.

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