Phytase activity in Aspergillus fumigatus isolates.

Extracellular phytase from Aspergillus fumigatus isolates was characterized and their genes were cloned and sequenced. Based on their banding pattern in SDS-PAGE all phytases were found to be glycosylated and have similar molecular mass. A correlation between lower optimum pH (4.0) and a higher optimum temperature (70 degrees C) was found in these enzymes. All enzymes characterized displayed a lower specific activity for phytic acid and were more susceptible to proteolytic degradation than the Aspergillus niger phytase that is now commercially available. DNA sequencing established almost no sequence variation in any of the genes and no correlation is evident between a specific amino acid sequence and any physicochemical and catalytic properties of the enzymes. Despite two of the isolates having identical deduced amino acid sequence, characterization of the enzymes encoded by these two identical genes revealed differences in both pH and temperature optimum. This suggests that differences in pH and temperature optimum in these four isolates of A. fumigatus may be due in part to subtle differences in posttranslational modification.

Molecular and physiological aspects of aflatoxin and sterigmatocystin biosynthesis by Aspergillus tamarii and A. ochraceoroseus.

Until recently, only three species (Aspergillus flavus, A. parasiticus and A. nomius) have been widely recognized as producers of aflatoxin. In this study we examine aflatoxin production by two other species, A. tamarii and A. ochraceoroseus, the latter of which also produces sterigmatocystin. Toxin-producing strains of A. tamarii and A. ochraceoroseus were examined morphologically, and toxin production was assayed on different media at different pH levels using thin layer chromatography and a densitometer. Genomic DNA of these two species was probed with known aflatoxin and sterigmatocystin biosynthesis genes from A. flavus, A. parasiticus and A. nidulans. Under the high stringency conditions, A. tamarii DNA hybridized to all four of the A. flavus and A. parasiticus gene probes, indicating strong similarities in the biosynthetic pathway genes of these three species. The A. ochraceoroseus DNA hybridized weakly to the A. flavus and A. parasiticus verB gene probe, and to two of the three A. nidulans probes. These data indicate that, at the DNA level, the aflatoxin and sterigmatocystin biosynthetic pathway genes for A. ochraceoroseus are somewhat different from known pathway genes.

Advances in phytase research.

Since its discovery in 1907, a complex of technological developments has created a potential $500 million market for phytase as an animal feed additive. During the last 30 years, research has led to increased use of soybean meal and other plant material as protein sources in animal feed. One problem that had to be overcome was the presence of antinutritional factors, including phytate, in plant meal. Phytate phosphorus is not digested by monogastric animals (e.g., hogs and poultry), and in order to supply enough of this nutrient, additional phosphate was required in the feed ration. Rock phosphate soon proved to be a cost-effective means of supplying this additional phosphorus, and the excess phytin phosphorus could be disposed of easily with the animals' manure. However, this additional phosphorus creates a massive environmental problem when the land's ability to bind it is exceeded. Over the last decade, numerous feed studies have established the efficacy of a fungal phytase, A. niger NRRL 3135, to hydrolyze phytin phosphorus in an animal's digestive tract, which benefits the animal while reducing total phosphorus levels in manure. The gene for phytase has now been cloned and overexpressed to provide a commercial source of phytase. This monomeric enzyme, a type of histidine acid phophatase (HAP), has been characterized and extensively studied. HAPs are also found in other fungi, plants, and animals. Several microbial and plant HAPs are known to have significant phytase activity. A second A. niger phytase (phyB), a tetramer, is known and, like phyA, has had its X-ray crystal structure determined. The model provided by this crystal structure research has provided an enhanced understanding of how these molecules function. In addition to the HAP phytase, several other phytases that lack the unique HAP active site motif RHGXRXP have been studied. The best known group of the non-HAPs is phytase C (phyC) from the genus Bacillus. While a preliminary X-ray crystallographic analysis has been initiated, no enzymatic mechanism has been proposed. Perhaps the pivotal event in the last century that created the need for phytase was the development of modern fertilizers after the Second World War. This fostered a transformation in agriculture and a tremendous increase in feed-grain production. These large quantities of cereals and meal in turn led to the transition of one segment of agriculture into "animal agriculture," with their its animal production capability. The huge volumes of manure spawned by these production units in time exceeded both the capacity of their crops and crop lands to utilize or bind the increased amount of phosphorus. Nutrient runoff from this land has now been linked to a number of blooms of toxin-producing microbes. Fish kills associated with these blooms have attracted public and governmental concern, as well as greater interest in phytase as a means to reduce this phosphorus pollution. Phytase research efforts now are focused on the engineering of an improved enzyme. Improved heat tolerance to allow the enzyme to survive the brief period of elevated temperature during the pelletization process is seen as an essential step to lower its cost in animal feed. Information from the X-ray crystal structure of phytase is also relevant to improving the pH optimum, substrate specificity, and enzyme stability. Several studies on new strategies that involve synergistic interactions between phytase and other hydrolytic enzymes have shown positive results. Further reduction in the production cost of phytase is also being pursued. Several studies have already investigated the use of various yeast expression systems as an alternative to the current production method for phytase using overexpression in filamentous fungi. Expression in plants is underway as a means to commercially produce phytase, as in biofarming in which plants such as alfalfa are used as "bioreactors," and also by developing plant cultivars that would produce enough transgenic phytase so that additional supplementation of their grain or meals is not necessary. Ultimately, transgenic poultry and hogs may produce their own digestive phytase. Another active area of current phytase research is expanding its usage. One area that offers tremendous opportunity is increasing the use of phytase in aquaculture. Research is currently centered on utilizing phytase to allow producers in this industry to switch to lower-cost plant protein in their feed formulations. Development of a phytase for this application could significantly lower production costs. Other areas for expanded use range from the use of phytase as a soil amendment, to its use in a bioreactor to generate specific myo-inositol phosphate species. The transformation of phytase into a peroxidase may lead to another novel use for this enzyme. As attempts are made to widen the use of phytase, it is also important that extended exposure and breathing its dust be avoided as prudent safety measures to avoid possible allergic responses. In expanding the use of phytase, another important consideration has been achieved. Conservation of the world's deposits of rock phosphate is recognized as important for future generations. Phosphorus is a basic component of life like nitrogen, but, unlike nitrogen, phosphorus does not have a cycle to constantly replenish its supply. It is very likely that the use of phytase will expand as the need to conserve the world's phosphate reserves increases.

The Aspergillus niger (ficuum) aphA gene encodes a pH 6.0-optimum acid phosphatase.

We have used the Aspergillus niger (An) aphA gene as a probe and cloned the A. ficuum (Af) SRRC 265 gene encoding an extracellular pH 6.0-optimum acid phosphatase (APase6) from a genomic library. The identity of the Af aphA gene was confirmed and its nucleotide (nt) sequence verified by comparing its deduced amino acid (aa) sequence to that of purified Af APase6. A comparison of the nt sequences of the An and Af genes suggested that errors were made in the previously reported An aphA sequence. Several regions of the An aphA were resequenced and the mistakes corrected. With its nt sequence corrected, the An aphA is nearly identical to the cloned Af gene encoding APase6, and in 90.4% agreement in the coding regions. Both genes have three conserved introns and when translated, both nt sequences code for a polypeptide of 614 aa. There is now evidence that the two cloned genes are homologous and code for acid phosphatases that are 96% identical.