Regulation of glutamine synthetase from the white button mushroom Agaricus bisporus

Expression of the urease gene of Agaricus bisporus: a tool for studying fruit body formation and post-harvest development

Fruit body initials of Agaricus bisporus contain high levels of urea, which decrease in the following developmental stages until stage 4 (harvest) when urea levels increase again. At storage, the high urea content may affect the quality of the mushroom, i.e. by the formation of ammonia from urea through the action of urease (EC 3.5.1.5). Despite the abundance of urea in the edible mushroom A. bisporus, little is known about its physiological role. The urease gene of A. bisporus and its promoter region were identified and cloned. The coding part of the genomic DNA was interrupted by nine introns as confirmed by cDNA analysis. The first full homobasidiomycete urease protein sequence obtained comprised 838 amino acids (molecular mass 90,694 Da, pI 5.8). An alignment with fungal, plant and bacterial ureases revealed a high conservation. The expression of the urease gene, measured by Northern analyses, was studied both during normal development of fruit bodies and during post-harvest senescence. Expression in normal development was significantly up-regulated in developmental stages 5 and 6. During post-harvest senescence, the expression of urease was mainly observed in the stipe tissue; expression decreased on the first day and remained at a basal level through the remaining sampling period.

Effect of Chalara longipes on decomposition of humic acids from Picea abies needle litter

The effect of the saprotrophic ascomycete Chalara longipes on the decomposition of humic acids was determined in a cultivation experiment. The fungus was incubated in liquid cultures in a full-strength (F system) and an organic nitrogen-free medium (F-N system), both amended with pure humic acids isolated from spruce forest litter. Fungal biomass production was highest in the F system with humic acids and lowest in the F-N system, the effect of organic nitrogen and humic acids being significant (p < 0.05). The presence of organic nitrogen seems to be essential for growth. The fungus utilized humic acids; molar mass distribution in media obtained by gel-permeation chromatography showed decrease in the humic acids fraction. Decolorization of the media reached 75 and 64 % in the F and F-N systems, respectively. The molar mass of humic acids probably also decreased as estimated from the increase in the absorbance A465/A665 (A4/A6) ratio. It is assumed that the utilization of humic acids may be mediated by the production of organic acids (as their components) and production of some oxidative enzymes.

The ornithine cycle enzyme arginase from Agaricus bisporus and its role in urea accumulation in fruit bodies

An extensive survey of higher fungi revealed that members of the family Agaricaceae, including Agaricus bisporus, accumulate substantial amounts of urea in their fruit bodies. An important role of the ornithine cycle enzymes in urea accumulation has been proposed. In this work, we present the cloning and sequencing of the arginase gene and its promoter region from A. bisporus. A PCR-probe based on fungal arginase was used to identify the A. bisporus arginase gene from a cDNA library. The arginase cDNA encodes a 311-aa protein which is most likely expressed in the cytosol. Expression of the cDNA in Escherichia coli was established as a His-tagged fusion protein. The arginase gene was used as a molecular marker to study expression and regulation during sporophore formation and postharvest development. The expression of the arginase gene was significantly up-regulated from developmental stage 3 onwards for all the tissues studied. A maximum of expression was reached at stage 6 for both stipe and cap tissue. In postharvest stages 5, 6 and 7 the level of expression observed was similar to normal growth stages 5, 6 and 7. A good correlation was found between arginase expression and urea content of stipe, velum, gills, cap and peel tissue. For all tissues the urea content decreased over the first four stages of development. From stage 4 onwards urea accumulated again except for stipe tissue where no significant changes were observed. The same trend was also observed for postharvest development, but the observed increase of urea in postharvest tissues was much higher.