Structural features of the glycogen branching enzyme encoding genes from aspergilli

Spatial heterogeneity of glycogen and its metabolizing enzymes in Aspergillus nidulans hyphal tip cells

Glycogen is a homopolymer of glucose and a ubiquitous cellular-storage carbon. This study investigated which Aspergillus nidulans genes are involved in glycogen metabolism. Gene disruptants of predicted glycogen synthase (gsyA) and glycogenin (glgA) genes accumulated less cellular glycogen than the wild type strain, indicating that GsyA and GlgA synthesize glycogen similarly to other eukaryotes. Meanwhile, gene disruption of gphA encoding glycogen phosphorylase increased the amount of glycogen to a higher degree than wild type during the stationary phase that accompanies carbon-source limitation. GFP-tagged GsyA and GphA were distributed in the cytosol and formed punctate and filamentous structures, respectively. Carbon starvation resulted in elongated GphA-GFP filaments and increased numbers of filaments. These structures were more frequently located in the basal regions of tip cells and adjacent cells than in the apical regions of tip cells. Cellular glycogen visualized by incorporation of a fluorescent glucose analog accumulated in cytoplasmic puncta that were more prevalent in the basal regions, a pattern similar to that seen for GsyA. The colocalization of glycogen and GsyA at punctate structures in tip and sub-apical cells likely represents the cellular machinery for synthesizing glycogen. More frequent colocalization in the basal, rather than tip cell apical regions indicated that tip cells have differentiated subcellular regions for glycogen synthesis. Our findings regarding glycogen, GsyA and GphA distribution evoke the spatial heterogeneity of glycogen metabolism in fungal hyphae.

Aspergillus niger genome-wide analysis reveals a large number of novel alpha-glucan acting enzymes with unexpected expression profiles

The filamentous ascomycete Aspergillus niger is well known for its ability to produce a large variety of enzymes for the degradation of plant polysaccharide material. A major carbon and energy source for this soil fungus is starch, which can be degraded by the concerted action of α-amylase, glucoamylase and α-glucosidase enzymes, members of the glycoside hydrolase (GH) families 13, 15 and 31, respectively. In this study we have combined analysis of the genome sequence of A. niger CBS 513.88 with microarray experiments to identify novel enzymes from these families and to predict their physiological functions. We have identified 17 previously unknown family GH13, 15 and 31 enzymes in the A. niger genome, all of which have orthologues in other aspergilli. Only two of the newly identified enzymes, a putative α-glucosidase (AgdB) and an α-amylase (AmyC), were predicted to play a role in starch degradation. The expression of the majority of the genes identified was not induced by maltose as carbon source, and not dependent on the presence of AmyR, the transcriptional regulator for starch degrading enzymes. The possible physiological functions of the other predicted family GH13, GH15 and GH31 enzymes, including intracellular enzymes and cell wall associated proteins, in alternative α-glucan modifying processes are discussed.

Analysis of expressed sequence tags from the fungus Aspergillus oryzae cultured under different conditions

We performed random sequencing of cDNAs from nine biologically or industrially important cultures of the industrially valuable fungus Aspergillus oryzae to obtain expressed sequence tags (ESTs). Consequently, 21 446 raw ESTs were accumulated and subsequently assembled to 7589 non-redundant consensus sequences (contigs). Among all contigs, 5491 (72.4%) were derived from only a particular culture. These included 4735 (62.4%) singletons, i.e. lone ESTs overlapping with no others. These data showed that consideration of culture grown under various conditions as cDNA sources enabled efficient collection of ESTs. BLAST searches against the public databases showed that 2953 (38.9%) of the EST contigs showed significant similarities to deposited sequences with known functions, 793 (10.5%) were similar to hypothetical proteins, and the remaining 3843 (50.6%) showed no significant similarity to sequences in the databases. Culture-specific contigs were extracted on the basis of the EST frequency normalized by the total number for each culture condition. In addition, contig sequences were compared with sequence sets in eukaryotic orthologous groups (KOGs), and classified into the KOG functional categories.