A trial of minimization of chromosome 7 in Aspergillus oryzae by multiple chromosomal deletions.

We aim to create an Aspergillus oryzae mutant with a highly reduced chromosome, but better growth, by eliminating the nonessential regions coding various dispensable functions for its better industrial use. In our previous study, we successfully determined the outline of essential and nonessential regions by constructing a series of large chromosomal deletions in A. oryzae chromosome 7. Based on these results, we here constructed two mutants, designated RkuAF7A and RkuAF7B, lacking 24.7 and 24% (725 and 705 kb) of wild type chromosome 7, respectively, using multiple large-scale chromosomal deletions in a recursive pyrG-mediated transformation system. Both showed higher amylase activity in DPY liquid medium and faster growth rate on malt agar medium relative to the parent strain. The two mutants also displayed soft fluffy hyphal morphology when grown in DPY liquid media. In addition, the gene expression profile obtained by DNA microarray indicated that although the deletion regions were fewer than 2% of the whole genome, the effect on whole gene expression exceeded 20%. Among these, the genes involved in secondary metabolism showed a relatively large change in their gene expression levels. Together, the constructed mutants showing better growth and potential usefulness is possibly suitable for further industrial use.