Differential expression of three alpha-galactosidase genes and a single beta-galactosidase gene from Aspergillus niger

A gene encoding a third alpha-galactosidase (AglB) from Aspergillus niger h as been cloned and sequenced. The gene consists of an open reading frame of 1,750 bp containing six introns. The gene encodes a protein of 443 amino a cids which contains a eukaryotic signal sequence of 16 amino acids and seve n putative N-glycosylation sites. The mature protein has a calculated molec ular mass of 48,835 Ha and a predicted pi of 4.6. An alignment of the AglB amino acid sequence with those of other alpha-galactosidases revealed that it belongs to a subfamily of alpha-galactosidases that also includes A. nig er AglA A. niger AglC belongs to a different subfamily that consists mainly of prokaryotic alpha-galactosidases. The expression of aglA, aglB, aglC, a nd lacA, the latter of which encodes an A. niger beta-galactosidase, has be en studied by using a number of monomeric, oligomeric, and polymeric compou nds as growth substrates. Expression of aglA is only detected on galactose and galactose-containing oligomers and polymers. The aglB gene is expressed on all of the carbon sources tested, including glucose. Elevated expressio n was observed on xylan, which could be assigned to regulation via XlnR, th e xylanolytic transcriptional activator. Expression of aglC was only observ ed on glucose, fructose, and combinations of glucose with xylose and galact ose. High expression of lacA was detected on arabinose, xylose, xylan, and pectin. Similar to aglB, the expression on xylose and xylan can be assigned to regulation via XlnR. All four genes have distinct expression patterns w hich seem to mirror the natural substrates of the encoded proteins.