Molecular Characterization and Expression Mechanism of Maltose-Utilization Gene Clusters in a Koji-Mold, Aspergillus oryzae

点击翻译按钮获取中文摘要

One candidate EST clone homologous to the yeast maltase gene (MAL62) was found in the Aspergillus oryzae EST database. When an A. oryzae genomic library was screened with the clone as a probe, two different gene clusters probably involved in maltose utilization have been isolated. One is consisted of maltase homologue itself, designated malT, and a gene highly homologous to the yeast maltose permease gene (MAL61), designated malP. In addition, a putative transcriptional regulator gene designated malR which has a typical zinc finger motif at N-terminus is located at downstream of the malT. The transcriptional analysis showed that the malP and malT genes within this MAL cluster were induced by maltose but not by glucose or glycerol whereas the malR gene was expressed constitutively. The manner of their expression is similar to the amylolytic genes. The malP gene was introduced to maltose permease deficient yeast and was proved to encode a protein with a capability of incorporating maltose. Disruption analyses of the malP and malR genes in A. oryzae showed that the malP encodes a major maltose transporter in this fungus and that the malR is required for the efficient expression of the malP and malT genes. Another gene cluster is extensively highly homologous to the putative sugar utilization gene cluster in Aspergillus parasiticus. This GLC cluster has genes encoding α-glucosidase (glcA), a sugar transporter (hxtA), and a transcriptional activator (sugR) and is located at one end of the aflatdxin biosynthetic gene cluster, as in A. parasiticus. However, putative genes homologous to the nadA and stcQ located immediately prior to the GLC cluster have mutations of deletion or frame-shift, and thus may be inactive. In addition, it was not observed so far that the genes contained in the GLC cluster are transcribed.