Control of the Nitrogen Isotope Composition of the Fungal Biomass: Evidence of Microbial Nitrogen Use Efficiency.

Changes in 15N/14N in the soil microbial biomass during nitrogen (N) mineralization have been hypothesized to influence 15N/14N in soil organic matter among ecosystem sites. However, a direct experimental test of this mechanism has not yet been performed. To evaluate the potential control of microbial N mineralization on the natural N isotope composition, we cultured fungi (Aspergillus oryzae) in five types of media of varying C:N ratios of 5, 10, 30, 50, and 100 for 4 d, and tracked changes in δ15N in the microbial biomass, NH4+, and dissolved organic N (DON: glycine) over the course of the experiment. High rates of NH4+ excretion from A. oryzae were accompanied by an increase in δ15N in the microbial biomass in low C:N media (i.e., C/N<30). In contrast, NH4+ was strongly retained in higher C/N treatments with only minor (i.e., <1 ‰) changes being detected in δ15N in the microbial biomass. Differences in δ15N in the microbial biomass were attributed to the loss of low-δ15N NH4+ in low, but not high C/N substrates. We also detected a negative linear correlation between microbial nitrogen use efficiency (NUE) and Δ15N (δ15N-biomass-δ15N-glycine). These results suggest an isotope effect during NH4+ excretion in relatively N-repleted environments in which microbial NUE is low, which may explain the vertical patterns of organic matter δ15N in soil profiles.

Heterologous expression and characterization of CpI, OcpA, and novel serine-type carboxypeptidase OcpB from Aspergillus oryzae.

In the genome of Aspergillus oryzae, 12 genes have been predicted to encode serine-type carboxypeptidases. However, the carboxypeptidase activities of the proteins encoded by these genes have not yet been confirmed experimentally. In this study, we have constructed three of these 12 genes overexpressing strains using Aspergillus nidulans and characterized their overproduced recombinant proteins. Of these three genes, one was previously named cpI; the other two have not been reported yet, and hence, we named them ocpA and ocpB. The recombinant proteins released amino acid residues from the C terminus of peptides, and the activity of the enzymes was inhibited by phenylmethylsulfonyl fluoride, indicating the enzymes to be serine-type carboxypeptidases. Recombinant OcpA, OcpB, and CpI were stable at 45 degrees C, 55 degrees C, and 55 degrees C, respectively, at a low pH. The enzymatic properties of recombinant OcpB were different from those of any reported serine-type carboxypeptidase. On the other hand, recombinant OcpA had similar enzymatic properties to A. oryzae carboxypeptidases O1 and O2. The DNA and N-terminal amino acid sequences of carboxypeptidases O1 and O2 from A. oryzae IAM2640 were similar to those of OcpA. Result of transcriptional analysis of ocpA, ocpB, and cpI suggest differences in transcriptional regulation between these genes.

The fungal hydrophobin RolA recruits polyesterase and laterally moves on hydrophobic surfaces.

When fungi grow on plant or insect surfaces coated with wax polyesters that protect against pathogens, the fungi generally form aerial hyphae to contact the surfaces. Aerial structures such as hyphae and conidiophores are coated with hydrophobins, which are surface-active proteins involved in adhesion to hydrophobic surfaces. When the industrial fungus Aspergillus oryzae was cultivated in a liquid medium containing the biodegradable polyester polybutylene succinate-coadipate (PBSA), the rolA gene encoding hydrophobin RolA was highly transcribed. High levels of RolA and its localization on the cell surface in the presence of PBSA were confirmed by immunostaining. Under these conditions, A. oryzae simultaneously produced the cutinase CutL1, which hydrolyses PBSA. Pre-incubation of PBSA with RolA stimulated PBSA degradation by CutL1, suggesting that RolA bound to the PBSA surface was required for the stimulation. Immunostaining revealed that PBSA films coated with RolA specifically adsorbed CutL1. Quartz crystal microbalance analyses further demonstrated that RolA attached to a hydrophobic sensor chip specifically adsorbed CutL1. Circular dichroism spectra of soluble-state RolA and bound RolA suggested that RolA underwent a conformational change after its adsorption to hydrophobic surfaces. These results suggest that RolA adsorbed to the hydrophobic surface of PBSA recruits CutL1, resulting in condensation of CutL1 on the PBSA surface and consequent stimulation of PBSA hydrolysis. A fluorescence recovery after photobleaching experiment on PBSA films coated with FITC-labelled RolA suggested that RolA moves laterally on the film. We discuss the novel molecular functions of RolA with regard to plastic degradation.