Asparaginase II of Saccharomyces cerevisiae: inactivation during the transition to stationary phase

The periplasmic enzyme, AnsB, of Shigella flexneri modulates bacterial adherence to host epithelial cells

S. flexneri strains, most frequently linked with endemic outbreaks of shigellosis, invade the colonic and rectal epithelium of their host and cause severe tissue damage. Here we have attempted to elucidate the contribution of the periplasmic enzyme, L-asparaginase (AnsB) to the pathogenesis of S. flexneri. Using a reverse genetic approach we found that ansB mutants showed reduced adherence to epithelial cells in vitro and attenuation in two in vivo models of shigellosis, the Caenorhabditis elegans and the murine pulmonary model. To investigate how AnsB affects bacterial adherence, we compared the proteomes of the ansB mutant with its wild type parental strain using two dimensional differential in-gel electrophoresis and identified the outer membrane protein, OmpA as up-regulated in ansB mutant cells. Bacterial OmpA, is a prominent outer membrane protein whose activity has been found to be required for bacterial pathogenesis. Overexpression of OmpA in wild type S. flexneri serotype 3b resulted in decreasing the adherence of this virulent strain, suggesting that the up-regulation of OmpA in ansB mutants contributes to the reduced adherence of this mutant strain. The data presented here is the first report that links the metabolic enzyme AnsB to S. flexneri pathogenesis.

Identification of an asparagine amidohydrolase from the filarial parasite Dirofilaria immitis

The nematode cuticle is a complex extracellular structure which is secreted by an underlying syncytium of hypodermal cells. Recent studies have demonstrated that the cuticle of parasitic nematodes is a dynamic structure with important absorptive, secretory, and enzymatic activities. In addition, the cuticle serves as a protective barrier against the host. A 48-h third stage larval Dirofilaria immitis cDNA library was immunoscreened with sera raised against larval cuticles. One clone, L3MC4 that reacted strongly with the anti-cuticle antisera was sequenced. The composite cDNA sequence comprises 2073 bp coding for a full-length protein of 590 amino acids. GenBank analysis showed that DiAsp had significant similarity to a Caenorhabditis elegans gene-product (54% identity) and to other asparaginases at the amino acid level. Escherichia coli-expressed recombinant DiAsp (rDiAsp) catalysed the hydrolysis of asparagine to aspartate and ammonia. Antibodies raised against D. immitis larval cuticles reacted with rDiAsp in immunoblots. This is the first report of identification of a cDNA clone encoding an asparaginase enzyme from a parasitic nematode.

Asparaginase II of Saccharomyces cerevisiae. GLN3/URE2 regulation of a periplasmic enzyme

The production of some extracellular enzymes is known to be negatively affected by readily metabolized nitrogen sources such as NH4+ although there is no consensus regarding the involved mechanisms. Asparaginase II is a periplasmic enzyme of Saccharomyces cerevisiae encoded by the ASP3 gene. The enzyme activity is not found in cells grown in either ammonia, glutamine, or glutamate, but it is found in cells that have been subjected to nitrogen starvation or have been grown on a poor source of nitrogen such as proline. In this report it is shown that the formation of this enzyme is dependent upon the functional GLN3 gene and that the response to nitrogen availability is under the control of the URE2 gene product. In this respect the expression of ASP3 is similar to the system that regulates the GLN1, GDH2, GAP1, and PUT4 genes that codes for glutamine synthetase, NAD-linked glutamate dehydrogenase, general amino-acid permease, and high affinity proline permease, respectively.

Asparaginase II of Saccharomyces cerevisiae: selection of four mutations that cause derepressed enzyme synthesis

A positive selection method was used to isolate four Saccharomyces cerevisiae mutations that cause derepressed synthesis of asparaginase II. The four mutations (and1, and2, and3, and4) were neither closely linked to each other nor linked to previously characterized mutations (asp3, asp6) which cause the complete loss of asparaginase II activity. One of the new mutations (and4) was shown to be allelic to gdh-CR, a pleiotropic mutation which causes derepressed synthesis of a number of enzymes of nitrogen catabolism.

Asparaginase II of Saccharomyces cerevisiae: positive selection of two mutations that prevent enzyme synthesis

A positive selection method, D-aspartic acid beta-hydroxamate resistance, was used to isolate Saccharomyces cerevisiae strains lacking the ability to synthesize asparaginase II. Of 100 such mutant strains, 93 exhibited mutations which were allelic with asp3, a previously characterized mutation. The other seven strains carried a new mutation, asp6. The asp6 mutation segregated 2:2 in asp6 X wild-type crosses and assorted from the asp3 mutation in asp6 X asp3 crosses. All seven asp6 mutant isolates reverted at a relatively high frequency, whereas the asp3 mutant isolates did not revert under the same conditions. Various independent asp3 isolates were mated to give heteroallelic diploids, which when sporulated and spread on D-asparagine medium yielded no recombinant strains.

Inactivation of cell-associated fructosyltransferase in Streptococcus salivarius

In stationary phase, 95% of the fructosyltransferase (FTase) activity of Streptococcus salivarius ATCC 25975 was found associated with the cells. Within the first 15 min after inoculation into fresh medium, the specific activity of cell-associated FTase decreased by 92% of its initial value. After this period of initial loss, the enzyme was synthesized during exponential growth until a maximum level equivalent to that present before inoculation was obtained. The inactivation of FTase was also demonstrated in a nongrowing system. Washed cell suspensions incubated at 37 degrees C in 200 mM potassium phosphate buffer (pH 6.5) containing 10 microM Cu2+ lost 80 to 95% of their FRase activity after 30 min. This loss could be prevented by the addition of histidine, cysteine, or Ca2+ to the suspension mixture. A factor(s) essential for the inactivation of cell-associated FTase could itself be preferentially inactivated by heating cells at 40 degrees C for periods of up to 3 h, or by storage of cells at 0 to 4 degrees C for several days in a low-ionic-strength, low-pH, potassium phosphate buffer. Treatment of cells with the N-acetylmuramidase enzyme M-1, in the presence of 0.5 M melezitose, resulted in the release of FTase from the cell. The released enzyme was recovered in the supernatant fraction after centrifugation at 160,000 x g for 90 min. Comparison of solubilized active and inactivated FTase preparations by polyacrylamide gel electrophoresis demonstrated that the inactivation of cell-associated FTase activity was associated with the loss of specific protein bands.