The Asc locus for resistance to Alternaria stem canker in tomato does not encode the enzyme aspartate carbamoyltransferase

Intragenic recombination generated two distinct Cf genes that mediate AVR9 recognition in the natural population of Lycopersicon pimpinellifolium

Resistance gene Cf-9 of cultivated tomato (Lycopersicon esculentum) confers recognition of the AVR9 elicitor protein of the fungal pathogen Cladosporium fulvum. The Cf-9 locus, containing Cf-9 and four homologs (Hcr9s), originates from Lycopersicon pimpinellifolium (Lp). We examined naturally occurring polymorphism in Hcr9s that confer AVR9 recognition in the Lp population. AVR9 recognition occurs frequently throughout this population. In addition to Cf-9, we discovered a second gene in Lp, designated 9DC, which also confers AVR9 recognition. Compared with Cf-9, 9DC is more polymorphic, occurs more frequently, and is more widely spread throughout the Lp population, suggesting that 9DC is older than Cf-9. The sequences of Cf-9 and 9DC suggest that Cf-9 evolved from 9DC by intragenic recombination between 9DC and another Hcr9. The fact that the 9DC and Cf-9 proteins differ in 61 aa residues, and both mediate recognition of AVR9, shows that in nature Hcr9 proteins with the same recognitional specificity can vary significantly.

Aspartate transcarbamylase from the deep-sea hyperthermophilic archaeon Pyrococcus abyssi: genetic organization, structure, and expression in Escherichia coli

The genes coding for aspartate transcarbamylase (ATCase) in the deep-sea hyperthermophilic archaeon Pyrococcus abyssi were cloned by complementation of a pyrB Escherichia coli mutant. The sequence revealed the existence of a pyrBI operon, coding for a catalytic chain and a regulatory chain, as in Enterobacteriaceae. Comparison of primary sequences of the polypeptides encoded by the pyrB and pyrI genes with those of homologous eubacterial and eukaryotic chains showed a high degree of conservation of the residues which in E. coli ATCase are involved in catalysis and allosteric regulation. The regulatory chain shows more-extensive divergence with respect to that of E. coli and other Enterobacteriaceae than the catalytic chain. Several substitutions suggest the existence in P. abyssi ATCase of additional hydrophobic interactions and ionic bonds which are probably involved in protein stabilization at high temperatures. The catalytic chain presents a secondary structure similar to that of the E. coli enzyme. Modeling of the tridimensional structure of this chain provides a folding close to that of the E. coli protein in spite of several significant differences. Conservation of numerous pairs of residues involved in the interfaces between different chains or subunits in E. coli ATCase suggests that the P. abyssi enzyme has a quaternary structure similar to that of the E. coli enzyme. P. abyssi ATCase expressed in transgenic E. coli cells exhibited reduced cooperativity for aspartate binding and sensitivity to allosteric effectors, as well as a decreased thermostability and barostability, suggesting that in P. abyssi cells this enzyme is further stabilized through its association with other cellular components.

Identification and isolation of the FEEBLY gene from tomato by transposon tagging

The Ac/Ds transposon system from maize was used for insertional mutagenesis in tomato. Marker genes were employed for the selection of plants carrying a total of 471 unique Ds elements. Three mutants were obtained with Ds insertions closely linked to recessive mutations: feebly (fb), yellow jim (yj) and dopey (dp). The fb seedlings produced high anthocyanin levels, developed into small fragile plants, and were insensitive to the herbicide phosphinothricin. The yj plants had yellow leaves as a result of reduced levels of chlorophyll. The dp mutants completely or partially lacked inflorescences. The fb and yj loci were genetically linked to the Ds donor site on chromosome 3. Reactivation of the Ds element in the fb mutants by crosses with an Ac-containing line resulted in restoration of the wild-type phenotypes. Plant DNA fragments flanking both sides of the Ds element in the fb mutant were isolated by the inverse polymerase chain reaction. Molecular analysis showed that phenotypic reversions of fb were correlated with excisions of Ds. DNA sequence analysis of Fb reversion alleles showed the characteristic Ds footprints. Northern and cDNA sequence analysis indicated that transcription of the FEEBLY (FB) gene was impeded by the insertion of Ds in an intron. Comparison of the predicted amino acid sequence of the FB protein with other database sequences indicated that FB is a novel gene.

Effects of lipids on nucleotide inhibition of wheat-germ aspartate transcarbamoylase: evidence of an additional level of control?

Wheat-germ aspartate transcarbamoylase, a monofunctional trimer, is strongly inhibited by uridine 5'-monophosphate (UMP), which shows kinetic interactions with the substrate, carbamoyl phosphate, suggesting a classical allosteric mechanism of regulation. Inhibition of the purified enzyme by UMP was amplified in the presence of a variety of ionic lipids at concentrations low enough to preclude denaturation. In the absence of UMP, most of these compounds had no kinetic effect or were slightly activating. Two phospholipids did not show the effect. In a homologous series of fatty acids (C6-C16), the potentiating effect was only seen with homologues greater than C8, reaching a maximum at C12. The effect of dodecanoate (C12) on kinetic cooperativity (UMP as variable ligand) was studied. At each of several fixed concentrations of carbamoyl phosphate, dodecanoate had a pronounced effect on the half-saturating concentration of UMP, which was reduced by about half in every case, indicating substantially tighter binding of UMP. However, dodecanoate had relatively little effect on the kinetic Hill coefficient for the cooperativity of UMP. The possible metabolic significance of these effects is discussed.

Sphingosine-related mycotoxins in plant and animal diseases

The AAL-toxins and fumonisins are a group of chemically related phytotoxic congeners produced by Alternaria alternata f. sp. lycopersici and Fusarium moniliforme, respectively, that also are widespread mycotoxins with important health implications. These mycotoxins, which bear a structural relationship to the sphingoid base, sphingosine, also incite maladies in animals ranging from neoplasms to renal, neural, and hepatic necrosis. A. alternata f. sp. lycopersici causes the Alternaria stem canker disease in tomatoes, while F. moniliforme causes pink ear rot of maize and is associated with post-harvest contamination of many different food staples. These toxins are potent inhibitors of ceramide synthase in plants and animals. Sphingoid bases are mediators of signal transduction leading to neoplasms and necrosis in animals. Significant inhibition of ceramide synthase in microsomal preparations of tomato occurs at 20 nM with an I50in the range of 35–40 nM for both AAL-toxin, TA, and fumonisin, FB1. In plants, specific alterations of physiological processes associated with cellular response to these toxins appears to be required for cell death. A net decrease in sucrose influx to treated leaves occurs within 4 h of AAL-toxin treatment. Untreated leaves of toxin-resistant and -sensitive isolines of tomato show significant differences in sucrose transport capacity. Exogenous application of sucrose transport inhibitors mimicked AAL-toxin symptoms and enhanced cell death in susceptible lines of tomato. Conversely, the accumulation of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACQ occurred in 1 h and increased rapidly during the next 6 h after exposure to AAL-toxin. ACC accumulation is followed by a burst in ethylene within 12 h. Application of specific inhibitors of ethylene synthesis or ethylene action results in a decrease in toxin-induced cell death. These toxins appear to be useful tools for defining biochemical and molecular features common to induced cell death in both plants and animals. Key words: AAL-toxins, fumonisins, mycotoxins, host-selective toxins, Alternaria stem canker, Alternaria alternata, Fusarium moniliforme.

Inheritance and genetic mapping of resistance to Alternaria alternata f. sp. lycopersici in Lycopersicon pennellii

The fungal pathogen Alternaria alternata f. sp. lycopersici produces AAL-toxins that function as chemical determinants of the Alternaria stem canker disease in the tomato (Lycopersicon esculentum). In resistant cultivars, the disease is controlled by the Asc locus on chromosome 3. Our aim was to characterize novel sources of resistance to the fungus and of insensitivity to the host-selective AAL-toxins. To that end, the degree of sensitivity of wild tomato species to AAL-toxins was analyzed. Of all members of the genus Lycopersicon, only L. cheesmanii was revealed to be sensitive to AAL-toxins and susceptible to fungal infection. Besides moderately insensitive responses from some species, L. pennellii and L. peruvianum were shown to be highly insensitive to AAL-toxins as well as resistant to the pathogen. Genetic analyses showed that high insensitivity to AAL-toxins from L. pennellii is inherited in tomato as a single complete dominant locus. This is in contrast to the incomplete dominance of insensitivity to AAL-toxins of L. esculentum. Subsequent classical genetics, RFLP mapping and allelic testing indicated that high insensitivity to AAL-toxins from L. pennellii is conferred by a new allele of the Asc locus.