Systems biology studies of Aspergilli - from sequence to science

The recent dawn of the new biological mindset called systems biology has put forth a new way of analyzing and understanding biology. Carried by the notion that no element of a cell is an island, systems biology takes a holistic approach, and attempts to understand life as systems that have co-evolved and not as a haphazardly compiled list of parts.

This has been made possible by the socalled genomic revolution — the sequencing of the genomic DNA of a rapidly increasing number of organisms — and the “omic” tecniques following in the wake of the genome projects: metabolomic, proteomic, and transcriptomic to mention a few. The recent publication of the genome sequences of several filamentous fungi of the Aspergillus species (Aspergilli), has, along with the accumulation of years of reductionist studies, been a catalyst for the application of systems biology to this interesting group of fungi.

Among the genome sequenced Aspergilli are a known human pathogen (Aspergillus fumigatus), a model organism for cellular mechanisms (Aspergillus nidulans) and two industrial workhorses (Aspergillus niger and Aspergillus oryzae). In the presented work, new analytical and computational tools have been designed and a systems biology approach has been applied to a wide range of issues.

These tools include the compilation of data from literature on A. niger enzymes to form a re-constructed metabolic network and model of metabolism, allowing assessment of the industrial production potential of metabolites from this fungus. Based on the network, a map of metabolism has been drawn in the notation of biochemistry text books and a web server built that allows the analysis of genome-scale data for a number of Aspergillus species.

Further efforts have produced a tri-Aspergillus species Affymetrix DNA microarray for expression analysis of the predicted genes of A. nidulans, A. niger and A. oryzae, and a case study of an advanced application of this chip identified a regulatory response conserved through evolution. In an application of the tools, a multi-disciplinary comparison of two genome sequenced strains of A. niger was conducted using sequence analysis, exo-metabolomics, transcriptomics and classical genetics.

With a special focus on the traits making one strain a high-yield citric acid producer and the other an efficient glucoamylase producer, a surprising number of co-enhancing factors were found on multiple levels of cellular metabolism. Transcriptome profiling coupled with metabolic modeling further allowed the charting of the genome-wide regulation of A. niger in response to ambient pH in the context of organic acid production and a plausible explanation for the evolution of the capabilities to be a high-yield producer of citric acid.

Furthermore, a detailed transcriptome profiling of all of the genes of A. niger predicted to code for extracellular enzymes was made using a new literature-based graphical tool for analysis of the degradation targets of the enzymes. In conclusion, the versatility of a systems biology approach to understanding Aspergillus physiology has been demonstrated through a number of studies with results relevant to biotechnological processes.