The PRS gene family in Saccharomyces cerevisiae consists of five genes each capable of encoding a 5-phosphoribosyl-1(␣)-pyrophosphate synthetase polypeptide. To gain insight into the functional organization of this gene family we have constructed a collection of strains containing all possible combinations of disruptions in the five PRS genes. Phenotypically these deletant strains can be classified into three groups: (i) a lethal phenotype that corresponds to strains containing a double disruption in PRS2 and PRS4 in combination with a disruption in either PRS1 or PRS3; simultaneous deletion of PRS1 and PRS5 or PRS3 and PRS5 are also lethal combinations; (ii) a second phenotype that is encountered in strains containing disruptions in PRS1 and PRS3 together or in combination with any of the other PRS genes manifests itself as a reduction in growth rate, enzyme activity, and nucleotide content; (iii) a third phenotype that corresponds to strains that, although affected in their phosphoribosyl pyrophosphate-synthesizing ability, are unimpaired for growth and have nucleotide profiles virtually the same as the wild type. Deletions of PRS2, PRS4, and PRS5 or combinations thereof cause this phenotype. These results suggest that the polypeptides encoded by the members of the PRS gene family may be organized into two functional entities. Evidence that these polypeptides interact with each other in vivo was obtained using the yeast twohybrid system. Specifically PRS1 and PRS3 polypeptides interact strongly with each other, and there are significant interactions between the PRS5 polypeptide and either the PRS2 or PRS4 polypeptides. These data suggest that yeast phosphoribosyl pyrophosphate synthetase exists in vivo as multimeric complex(es).The enzyme 5-phosphoribosyl-1(␣)-pyrophosphate synthetase (ATP:D-ribose-5-phosphate pyrophosphotransferase; EC 2.7.6.1) (PRS) 1 catalyzes the reaction at a key junction in intermediary metabolism. PRS transfers the pyrophosphate moiety released from ATP to ribose-5-phosphate, thus giving rise to phosphoribosyl-pyrophosphate (PRPP) (1), and the enzyme therefore directs ribose-5-phosphate from energy generated by the pentose phosphate pathway to the important biosynthetic intermediate PRPP. PRPP is a precursor for the production of purine, pyrimidine, and pyridine nucleotides and the amino acids histidine and tryptophan (2). PRPP is required for both the de novo and the salvage pathways of nucleotide metabolism (3). It has been shown that in Mycobacterium spp. PRPP is also required for the biosynthesis of polyprenylphosphate pentoses that contribute to the arabinosyl residues of the cell wall (4).PRS genes have been cloned and sequenced from a variety of organisms; bacteria (5-9), mycoplasma (10), and protozoa (11) each contain apparently one PRS gene. In nematodes (12, 13) and the yeast Schizosaccharomyces pombe (14, 15) two PRS genes have been found so far, whereas in Spinacia oleracea four PRS cDNAs have been identified (16). PRS genes have also been cloned in rat (17-20) and human (21...

While recent pepino mosaic virus (PepMV; species Pepino mosaic virus, genus Potexvirus, family Alphaflexiviridae) epidemics seem to be predominantly caused by isolates of the CH2 strain, PepMV epidemics in intensive tomato crops in Spain are caused by both CH2 and EU isolates that co-circulate, representing a challenge in terms of control, including cross-protection. In this work, we hypothesized that mixed infections with two mild isolates of the EU and CH2 strains (PepMV-Sp13 and -PS5, respectively) may be useful in PepMV cross-protection in Spanish epidemics, providing protection against a broad range of aggressive isolates. Thus, we performed a range of field trials and an experimental evolution assay to determine the phenotypic and genetic stability of PepMV-Sp13 and -PS5 mixed infections, as well as their cross-protective efficiency. Our results showed that: (i) the phenotype of PepMV-Sp13 and -PS5 mixed infections was mild and did not change significantly when infecting different tomato cultivars or under different environmental conditions in Spain, (ii) PepMV-Sp13 and -PS5 mixed infections provided more efficient protection against two aggressive EU and CH2 isolates than single infections, and (iii) PepMV-Sp13 and -PS5, either in single or in mixed infections, were less variable than other two PepMV isolates occurring naturally in PepMV epidemics in Spain.

Pea (Pisum sativum L.) bacteroids produced by Rhizobium leguminosarum bv. viciae UPM791 synthesize a membrane-bound (NiFe) hydrogenase which oxidizes H 2 arising from the nitrogen fixation process in root nodules. Synthesis of the active enzyme requires the products of the structural genes hupSL and an array of accessory proteins from at least 15 additional genes, including the gene cluster hypABFCDE, likely involved in nickel metabolism. Unlike the hupSL genes, which are expressed only in symbiosis, the hypBFCDE operon was also activated in vegetative cells in response to low pO 2 in the culture medium. In microaerobic cells and in bacteroids, transcription of the hypBFCDE operon occurred from a promoter, P 5b , with a transcription initiation site located 190 bp upstream of the ATG start codon of hypB, within the coding sequence of hypA. Transcription start site 5b was preceded by an Fnr box (anaerobox), 5-TTGAgccatgTCAA-3, centered at position ؊39.5. Expression of the P 5b promoter in the heterologous Rhizobium meliloti bacterial host was dependent on the presence of an active fixK gene. A 2.6-kb EcoRI fragment was isolated from an R. leguminosarum bv. viciae UPM791 gene bank by complementing an R. meliloti FixK ؊ mutant. Sequencing of this DNA fragment identified an fnrN gene, and cassette insertion mutagenesis demonstrated that R. leguminosarum bv. viciae fnrN is able to replace the R. meliloti fixK gene for activation of both the R. leguminosarum bv. viciae hypBFCDE operon and the R. meliloti fix genes. However, bacteroids from a genomic FnrN ؊ mutant of R. leguminosarum bv. viciae exhibited wild-type levels of hydrogenase activity. Microaerobic expression of P 5b was reduced to ca. 50% of the wild-type level in the FnrN ؊ mutant. These results indicate that hyp gene expression escapes mutagenesis of the fnrN gene and suggest the existence of a second fnr-like gene in R. leguminosarum bv. viciae. Southern blot analysis with an fnrN internal probe revealed the presence of a second genomic region with homology to fnrN.A few strains of Rhizobium leguminosarum bv. viciae have been found to synthesize, in symbiosis with peas, an H 2 uptake (hup) system that recycles hydrogen evolved during the nitrogen reduction process in root nodules (2). In R. leguminosarum bv. viciae UPM791, the genetic determinants for this H 2 uptake system are clustered in a 20-kb DNA region which has been isolated in cosmid pAL618 (21). About 15 kb of this region have been sequenced, and 17 genes closely linked and oriented in the same direction were identified (14,15,31,33). The first two genes of the cluster, hupS and hupL, encode the structural polypeptides of a dimeric (NiFe)-type hydrogenase (14) and belong to an operon that also includes four additional genes, hupCDEF (15). The remaining genes, named hupG, H, I, J, and K and hypA, B, F, C, D, and E, are located downstream of hupF (31, 33). These genes encode hydrogenase accessory proteins that are required for H 2 oxidation in pea bacteroids, but their specific molecular functions...

Rhizobium leguminosarum bv. viciae UPM791 contains a second copy of the fnrN gene, which encodes a redox-sensitive transcriptional activator functionally homologous to Escherichia coli Fnr. This second copy (fnrN2) is located in the symbiotic plasmid, while fnrN1 is in the chromosome. Isolation and sequencing of the fnrN2 gene revealed that the deduced amino acid sequence of FnrN2 is 87.5% identical to the sequence of FnrN1, including a conserved cysteine-rich motif characteristic of Fnr-like proteins. Individual R. leguminosarum fnrN1 and fnrN2 mutants exhibited a Fix ؉ phenotype and near wild-type levels of nitrogenase and hydrogenase activities in pea (Pisum sativum L.) nodules. In contrast, an fnrN1 fnrN2 double mutant formed ineffective nodules lacking both nitrogenase and hydrogenase activities. Unlike the wild-type strain and single fnrN1 or fnrN2 mutants, the fnrN1 fnrN2 double mutant was unable to induce micro-oxic or bacteroid activation of the hypBFCDEX operon, which encodes proteins essential for hydrogenase synthesis. In the search for symbiotic genes that could be controlled by FnrN, a fixNOQP operon, putatively encoding a micro-oxically induced, bacteroid-specific cbb 3 -type terminal cytochrome oxidase, was isolated from strain UPM791 and partially sequenced. The fixNOQP operon was present in a single copy located in the symbiotic plasmid, and an anaerobox was identified in the fixN promoter region. Consistent with this, a fixNOQP-lacZ fusion was shown to be highly induced in micro-oxic cells of the wild-type strain. A high level of micro-oxic induction was also observed in single fnrN1 and fnrN2 mutants, but no detectable induction was observed in the fnrN1 fnrN2 double mutant. The lack of expression of fixNOQP in the fnrN1 fnrN2 double mutant is likely to cause the observed Fix ؊ phenotype. These data demonstrate that, contrary to the situation in other rhizobia, FnrN controls both hydrogenase and nitrogenase activities of R. leguminosarum bv. viciae UPM791 in the nodule and suggest that this strain lacks a functional fixK gene.

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