Yeast inoculum was prepared on YPD agar (10 g lÀ1 yeast extract, 20 g lÀ1 peptone, 20 g lÀ1 dextrose, and 15 g lÀ1 agar) and incubated at 28 C for 24 h. One loop of inoculum was transferred into 5 ml of YPD broth, amended with 0.1 % (w/v) L-tryptophan