Chitin synthetase activity and chitin formation in conidia of Aspergillus nidulans during germination and the effect of cycloheximide and 5′-fluorouracil

Abstract

Conidia from Aspergillus nidulans were germinated in a fermenter over a period of 9 h. Germination occurred between 3 and 7.5 h. Chitin synthetase in both active and zymogen (trypsin-activated) forms was assayed in wall and membrane fractions of cell homogenates. Activities in dormant spores were very low. Prior to germ tube emergence (1.5–4.5 h), active and zymogen forms of the enzyme both increased greatly, at a rate in excess of overall increases in protein and dry weight. This was correlated with a steep rise in the chitin-glucosamine content of the cell wall, beginning at germ tube emergence (3 h onward). Incorporation of labeled N-acetylglucosamine indicated the existence of chitin turnover during the earliest stages of germination. Cycloheximide (100 mg/liter blocked all development of conidia, and there was no increase in chitin synthetase activity. 5′-fluorouracil (2 mmol/liter) inhibited germination but permitted large increases in chitin synthetase activity and the chitin content of the walls. The results suggest that the chitin synthetase zymogen was synthesized de novo during germination, initially directed by mRNA preformed during conidiogenesis.