Tyramine, ␤-phenylethylamine, tryptamine, and octopamine are biogenic amines present in trace levels in mammalian nervous systems. Although some ''trace amines'' have clearly defined roles as neurotransmitters in invertebrates, the extent to which they function as true neurotransmitters in vertebrates has remained speculative. Using a degenerate PCR approach, we have identified 15 G protein-coupled receptors (GPCR) from human and rodent tissues. Together with the orphan receptor PNR, these receptors form a subfamily of rhodopsin GPCRs distinct from, but related to the classical biogenic amine receptors. We have demonstrated that two of these receptors bind and͞or are activated by trace amines. The cloning of mammalian GPCRs for trace amines supports a role for trace amines as neurotransmitters in vertebrates. Three of the four human receptors from this family are present in the amygdala, possibly linking trace amine receptors to affective disorders. The identification of this family of receptors should rekindle the investigation of the roles of trace amines in mammalian nervous systems and may potentially lead to the development of novel therapeutics for a variety of indications.

Turnip crinkle virus (TCV) inoculation onto TCV-resistant Arabidopsis leads to a hypersensitive response (HR) controlled by the dominant gene HRT. HRT is a member of the class of resistance (R) genes that contain a leucine zipper, a nucleotide binding site, and leucine-rich repeats. The chromosomal position of HRT and its homology to resistance gene RPP8 and two RPP8 homologs indicate that unequal crossing over and gene conversion may have contributed to HRT evolution. RPP8 confers resistance to an oomycete pathogen, Peronospora parasitica. Despite very strong similarities within the HRT/RPP8 family, HRT and RPP8 are specific for the respective pathogens they detect. Hence, the HRT/RPP8 family provides molecular evidence that sequence changes between closely related members of multigene families can generate novel specificities for radically different pathogens. Transgenic plants expressing HRT developed an HR but generally remained susceptible to TCV because of a second gene, RRT, that regulates resistance to TCV. However, several transgenic plants that overexpressed HRT produced micro-HRs or no HR when inoculated with TCV and were resistant to infection. Expression of the TCV coat protein gene in seedlings containing HRT resulted in massive necrosis and death, indicating that the avirulence factor detected by the HRT-encoded protein is the TCV coat protein.

Turnip crinkle virus (TCV) inoculation onto TCV-resistant Arabidopsis leads to a hypersensitive response (HR) controlled by the dominant gene HRT . HRT is a member of the class of resistance ( R ) genes that contain a leucine zipper, a nucleotide binding site, and leucine-rich repeats. The chromosomal position of HRT and its homology to resistance gene RPP8 and two RPP8 homologs indicate that unequal crossing over and gene conversion may have contributed to HRT evolution. RPP8 confers resistance to an oomycete pathogen, Peronospora parasitica . Despite very strong similarities within the HRT/RPP8 family, HRT and RPP8 are specific for the respective pathogens they detect. Hence, the HRT/ RPP8 family provides molecular evidence that sequence changes between closely related members of multigene families can generate novel specificities for radically different pathogens. Transgenic plants expressing HRT developed an HR but generally remained susceptible to TCV because of a second gene, RRT , that regulates resistance to TCV. However, several transgenic plants that overexpressed HRT produced micro-HRs or no HR when inoculated with TCV and were resistant to infection. Expression of the TCV coat protein gene in seedlings containing HRT resulted in massive necrosis and death, indicating that the avirulence factor detected by the HRT -encoded protein is the TCV coat protein. INTRODUCTIONTo survive, plants must defend themselves against numerous pathogens. Some defenses are constitutive, such as various preformed antimicrobial compounds (Osbourn, 1996), whereas others are activated by recognition of pathogens (Hammond-Kosack and Jones, 1996;Yang et al., 1997). The key players in this recognition process include the product of a dominant or semidominant resistance ( R ) gene present in the plant and the corresponding dominant avirulence (Avr) factor encoded by or derived from the pathogen. Recognition of the Avr factor by the host plant initiates one or more signal transduction pathways that activate various plant defenses and thus compromise the ability of the pathogen to colonize the plant. In this gene-for-gene interaction, the prevailing thought is that the R protein either acts directly as the receptor of the Avr factor (Ellingboe, 1980; Bent, 1996;Yang et al., 1997) or recognizes the Avr factor indirectly through a coreceptor (Dixon et al., 1998). To date, direct interaction between an R protein and an Avr factor has been demonstrated only for the tomato Pto and the Pseudomonas syringae AvrPto proteins (Scofield et al., 1996;Tang et al., 1996) and between the rice Pi-ta and the Magnaporthe grisea AvrPita proteins (Jia et al., 1999).An array of R genes that provide protection against viruses, bacteria, fungi, and oomycetes has been cloned from both monocots and dicots during the past 6 years (Staskawicz et al., 1995; Bent, 1996; Baker et al., 1997; DeWit, 1997). Many contain a nucleotide binding site (NBS). Often located closer to the N terminus of the R protein is either a leucine zipper or a TIR domain, which is sim...

Phosphoenolpyruvate carboxylase (PEPC) plays a key role in N2 fixation and ammonia assimilation in legume root nodules. The enzyme can comprise up to 2% of the soluble protein in root nodules. We report here the isolation and characterization of a cDNA encoding the nodule-enhanced form of PEPC. Initially, a 2945 bp partial-length cDNA was selected by screening an effective alfalfa nodule cDNA library with antibodies prepared against root nodule PEPC. The nucleotide sequence encoding the N-terminal region of the protein was obtained by primer-extension cDNA synthesis and PCR amplification. The complete amino acid sequence of alfalfa PEPC was deduced from these cDNA sequences and shown to bear striking similarity to other plant PEPCs. Southern blots of alfalfa genomic DNA indicate that nodule PEPC is a member of a small gene family. During the development of effective root nodules, nodule PEPC activity increases to a level that is 10- to 15-fold greater than that in root and leaf tissue. This increase appears to be the result of increases in amount of enzyme protein and PEPC mRNA. Ineffective nodules have substantially less PEPC mRNA, enzyme protein and activity than do effective nodules. Maximum expression of root nodule PEPC appears to be related to two signals. The first signal is associated with nodule initiation while the second signal is associated with nodule effectiveness. Regulation of root nodule PEPC activity may also involve post-translational processes affecting enzyme activity and/or degradation.

Aspartate aminotransferase (AAT) is a key plant enzyme affecting nitrogen and carbon metabolism, particularly in legume root nodules and leaves of C4 species. To ascertain the molecular genetic characteristics and biochemical regulation of AAT, we have isolated a cDNA encoding the nodule-enhanced AAT (AAT-2) of alfalfa (Medicago sativa L.) by screening a root nodule cDNA expression library with antibodies. Complementation of an Escherichia coRi AAT mutant with the alfalfa nodule AAT-2 cDNA verified the identity of the clone. The deduced amino acid sequence of alfalfa AAT-2 is 53 and 47% identical to animal mitochondrial and cytosolic AATs, respectively. The deduced molecular mass of AAT-2 is 50,959 daltons, whereas the mass of purified AAT-2 is about 40 kilodaltons as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the protein's N-terminal domain (amino acids 1-59) contains many of the characteristics of plastid-targeting peptides. We postulate that AAT-2 is localized to the plastid. Southem blot analysis suggests that AAT-2 is encoded by a small, multigene family. The expression of AAT-2 mRNA in nodules is severalfold greater than that in either leaves or roots. Northem and western blots showed that expression of AAT activity during effective nodule development is accompanied by a sevenfold increase in AAT-2 mRNA and a comparable increase in enzyme protein. By contrast, plant-controlled ineffective nodules express AAT-2 mRNA at much lower levels and have little to no AAT-2 enzyme protein. Expression of root nodule AAT-2 appears to be regulated by at least two events: the first is independent of nitrogenase activity; the second is associated with nodule effectiveness. AAT2 (EC 2.6.1.1) catalyzes the reversible reaction, glutamate + oxaloacetate <-+ aspartate + a-ketoglutarate. The '