Chemical Composition of Wild-type and Mutant Aspergillus nidulans Cell Walls. The Nature of Polysaccharide and Melanin Constituents

Abstract

S U M M A R YChitin and a P-linked glucan were the major chemical components of Aspergillus nidulans cell walls. Other monomeric residues identified in enzymic and acid hydrolyses of whole cell walls and cell-wall fractions included galactose, mannose, glucuronic acid and galactosamine. The P-glucan contained (I -+ 3) and ( I -+ 6) linkages and was two-thirds digested by an eX0-P-D-I ,3 glucanase prepared from a cell-wall lysing Streptomyces species. An a-glucan was identified as a cell-wall component and it also contained ( I -+ 3) linkages. This latter polysaccharide was distinguishable from nigeran (an a-1,3; a-1,4 glucan present in other Aspergillus species) by infrared spectroscopy and by its low susceptibility to hydrolysis by an endo-z-I ,3 ; a-1 ,4 glucan glucanohydrolase. Both glucans were alkali-soluble, but the P-glucan was completely solubilized only after acid extraction of the wall. The N-acetylglucosamine to galactosamine ratio in the A. nidulans cell wall was 1-32 and the two hexosamines were shown to be constituents of distinct polymers. The remaining cell wall was accounted for by protein, lipids, readily extractable and bound, and, in the wild-type, melanin.The melanin was distributed throughout the cell wall but was associated particularly with the chitin fraction. The pigment has been partially characterized chemically and contains indolic residues ; this result does not substantiate earlier views that indolic melanins are peculiar to the animal kingdom. Melanin appears to be a finite heteropolymer both in terms of its molecular size and its chemistry. I N T R O D U C T I O NA positive correlation has been demonstrated in several species between the presence of melanin, or melanin-like pigments, in the cell walls of fungi and resistance to microbial and enzymatic lysis (Potgieter & Alexander, 1966;Bloomfield &. Alexander, 1967). Furthermore, studies on Aspergillus nidulans (Kuo & Alexander., 1967) established that the resistance to lysis was affected by the melanin concentration in the cell wall. The mechanism by which melanin may exert its antilytic effect in this fungus has recently been discussed (Bull, 1g7oa).As an essential prelude to investigations of the melanin effect the cells walls of' Aspergillus nidulans were examined chemically with reference to the nature of' * (Bull & Faulkner, 1964. The two strains were distinguished by the presence and absence respectively of melanin in their cell walls. Stock cultures were maintained on a glucose-salts agar medium and 10 1. batch cultures for cell-wall preparation were grown in a liquid medium of similar composition (Carter & Bull, 1969; Bull, 197oa).Cell walls of Aspergillus nidulans I 3 . I . OL were degraded extensively by a complex of lytic enzymes synthesized by a Streptomyces species. The lytic organism was isolated from soil enriched with Phytophthora megasterium mycelium (Wang, I 964). Cultures of the streptomycete were maintained on a mineral salts agar containing 2 % (w/v) wet wt of blended Aspergillus oryzae mycelium (supplied...

Purification and characterisation of a sulphur rich melanin from edible mushroom Termitomyces albuminosus Heim

“…According to Bull (1970), in Aspergillus nidulans melanin pigment varied in composition with response to growth medium and the most significant finding was the widely varying nitrogen content of the melanin in response to the growth medium. Bull (1970) reported percentage composition of melanin in Czapek Dox Medium as C = 56.40%, H = 6.55%, and N = 3.92–1.78% (on addition of DOPA & Catechol), indicating that melanin composition can vary from medium to medium. High sulphur content of melanin in Termitomyces is possible due to availability of sulphur-containing amino acids and sulphite reductase enzymes.…”

Role of chitin synthase genes in Fusarium oxysporum

“…F. oxysporum has also been reported as an emerging opportunistic human pathogen in immunocompromised patients (Vartivarian et al, 1993 et al, 1999) and 20 % in Aspergillus spp. (Bull, 1970), compared to only 1-2 % in the yeast S. cerevisiae (Bulawa, 1993). This difference may explain the high number of structural chs genes identified in filamentous fungal species such as A. fumigatus, A. nidulans and N. crassa, in contrast to only three genes encoding the catalytic subunits reported in S. cerevisiae (Roncero, 2002) and four genes in Candida albicans (Munro & Gow, 2001).…”

The decomposition of ectomycorrhizal fungal necromass

“…While the chemical structure of these compounds is poorly understood, they are known to be generally composed of phenolic and indolic monomers (Bull, 1970;Bell and Wheeler, 1986;Butler and Day, 1998a). Fungi produce four classes of melanin that vary in their precursors and biosynthetic pathways.…”