IPT1-independent sphingolipid biosynthesis and yeast inhibition by syringomycin E and plant defensin DmAMP1

Both bacterial cyclic lipodepsipeptide syringomycin E and plant defensin DmAMP1 were shown previously to require expression of the yeast gene IPT1 for fungicidal action against Saccharomyces cerevisiae. IPT1 encodes a sphingolipid biosynthetic pathway glycotransferase that produces the terminal sphingolipid mannosyldiinositolphosphoceramide. However, when grown in half-strength potato dextrose medium, an ipt1 deletion mutant of S. cerevisiae was observed to be sensitive to syringomycin E and DmAMP1 and to produce small amounts of mannosyldiinositolphosphoceramide. These results show that the terminal sphingolipid but not IPT1 expression is required for fungicidal activity, and they suggest an IPT1-independent route for mannosyldiinositolphosphoceramide biosynthesis.