Impaired PRPP-synthesizing capacity compromises cell integrity signalling in Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, PRS genes comprise a family of five paralogous genes. Previously, it has been shown that in the cell the gene products are organized into two interacting complexes, one of which is a heterodimer and the other a heterotrimer. Here, it has been demonstrated that in addition to supplying the cell with the key metabolic intermediate PRPP [5-phospho-D-ribosyl-1(alpha)-pyrophosphate], the gene products contribute to the maintenance of cell integrity. Specifically, the phosphorylation of Rlm1, one of the end points of the cell integrity signalling pathway, is significantly impaired following deletion of any one of the PRS genes, in particular PRS1 and PRS3. This is reflected in changes in the expression of the alternative 1,3-beta-glucan synthase catalytic subunit, Fks2, as measured by its promoter activity. Yeast two-hybrid analysis has shown that Prs1, specifically the non-homologous region, NHR1-1 and Prs3, and to a lesser extent Prs2 and Prs4, interact with the MAPK (mitogen-activated protein kinase) of the cell integrity pathway, Slt2. When PRS1 is lacking, the basal level of phosphorylation of Slt2 is increased. Furthermore, prs1Delta and prs3Delta strains have an increased chitin content under normal growth conditions. alpha-Factor sensitivity and Calcofluor White resistance associated with the lack of Prs1 and Prs3 corroborate the involvement of these two gene products in cell integrity signalling. It is postulated that Prs polypeptides play a significant role in the remodelling of the cell wall and may have a direct involvement in cell integrity signalling.

[Inherited disorders of uric acid metabolism--classification, enzymatic- and DNA-diagnosis].

Uric acid is the end product of purine metabolism in human. Then, the enzymatic abnormalities, concerning purine metabolism, cause disorders of uric acid metabolism including hyperuricemia and hypouricemia. The superactivity of 5-phosphoribosyl-pyrophosphate (PRPP) synthetase and deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) caused hyperuricemia. In glycogen storage diseases of type I, III, V, and VII, decreased energy supply induces hyperuricemia by accelerating ATP degradation. Deficiencies of xanthine oxidase (XO), purine nucleoside phosphorylase (PNP), and PRPP were reported causing hypouricemia. Many methods for DNA-diagnosis were developed including Southern blot, Northern blot, PCR-SSCP (polymerase chain reaction-single strand conformation polymorphism), PCR-RFLP (restriction fragment length polymorphism), and allele specific oligonucleotide hybridization etc.

Phosphoribosyl diphosphate synthetase-independent NAD de novo synthesis in Escherichia coli: a new phenotype of phosphate regulon mutants.

Phosphoribosyl diphosphate-lacking (delta prs) mutant strains of Escherichia coli require NAD, guanosine, uridine, histidine, and tryptophan for growth. NAD is required by phosphoribosyl diphosphate-lacking mutants because of lack of one of the substrates for the quinolinate phosphoribosyltransferase reaction, an enzyme of the NAD de novo pathway. Several NAD-independent mutants of a host from which prs had been deleted were isolated; all of them were shown to have lesions in the pstSCAB-phoU operon, in which mutations lead to derepression of the Pho regulon. In addition NAD-independent growth was dependent on a functional quinolinate phosphoribosyltransferase. The prs suppressor mutations led to the synthesis of a new phosphoryl compound that may act as a precursor for a new NAD biosynthetic pathway. This compound may be synthesized by the product of an unknown phosphate starvation-inducible gene of the Pho regulon because the ability of pst or phoU mutations to suppress the NAD requirement requires PhoB, the transcriptional activator of the Pho regulon.