Effects of intron retention on properties of β-glucosidase in Aspergillus niger

Intron retention, one of the major types of alternative splicing in plants and animals, has also been reported existing in filamentous fungi's glycoside hydrolases. In this study, an intron-retained β-glucosidase gene transcript (bgl1B) from A. niger B2 strain was obtained. Compared with the normally spliced transcript bgl1A, bgl1B had an extra 51bp insertion, which was confirmed to be the sixth (the last) intron of this β-glucosidase gene. The bgl1A and bgl1B were expressed in Pichia pastoris and the purified enzymes were used to compare their catalytic properties. The results showed that the intron retention didn't impair the catalytic function. Instead, the intron-retained enzyme BGL1B had a better thermostability with a higher optimal temperature and a longer half-life under 50 °C. Also it exhibited a little higher k_cat for 4-nitrophenyl-β-d-glucopyranoside (PNPG) and a noticeable higher hydrolysis efficiency towards geniposide. This work suggested that the β-glucosidase gene in A. niger most likely underwent an alternative splicing presented as intron retention type, and intron retention might be a source of enzyme diversity in fungi.