The antifungal activity of radicicol against Mucor flavus IFO 9560 was inve
stigated, Radicicol induced bursting of spores during germination and morph
ological changes of the mycelial tip such as overbranching and swelling dur
ing exponential growth. In addition, radicicol showed a dose-dependent inhi
bitory effect on spore germination. Radicicol also inhibited the incorporat
ion of radioactive precursors into DNA, RNA, protein, and chitin fractions
by 20-30%, but not into the lipid fraction. There were no inhibitory effect
s on either endogenous or exogenous cellular respiration, Moreover, leakage
of UV-absorbing, phenol sulfate-positive, or folin reagent-positive materi
als from the mycelia was not observed at an early stage of growth inhibitio
n. On the other hand, kinetic studies of chitin synthase in the untreated m
ycelia revealed that radicicol noncompetitively inhibited the enzyme at K-i
of 87 mu M. Furthermore, upon incubation of the normal mycelia with radici
col in 50 mM KH2PP4-NaOH buffer (pH 6.5) containing 10 mM MgCl2, chitin syn
thase from the mycelia was inactivated gradually at first, and completely a
fter 24-h incubation. These results suggested that radicicol exhibits the a
ntifungal activity by disturbing cen wall biosynthesis through the inactiva
tion of chitin synthase. However, at an early stage of growth inhibition, r
adicicol was thought to affect cellular function including nucleic acid and
protein syntheses, in addition to the reversible noncompetitive inhibition
of chitin synthase.