CHARACTERIZATION OF ZEA-MAYS ENDOSPERM C-24 STEROL METHYLTRANSFERASE - ONE OF 2 TYPES OF STEROL METHYLTRANSFERASE IN HIGHER-PLANTS

We report the characterization of a higher-plant C-24 sterol methyltra nsferase by yeast complementation. A Zea mays endosperm expressed sequ ence tag (EST) was identified which, upon complete sequencing, showed 46% identity to the yeast C-24 methyltransferase gene (ERG6) and 75% a nd 37% amino acid identity to recently isolated higher-plant sterol me thyltransferases from soybean and Arabidopsis, respectively. When plac ed under GAL4 regulation, the Z. mays cDNA functionally complemented t he erg6 mutation, restoring ergosterol production and conferring resis tance to cycloheximide. Complementation was both plasmid-dependent and galactose-inducible. The Z. mays cDNA clone contains an open reading frame encoding a 40 kDa protein containing motifs common to a large nu mber of S-adenosyl-L-methionine methyltransferases (SMTs). Sequence co mparisons and functional studies of the maize, soybean and Arabidopsis cDNAs indicates two types of C-24 SMTs exist in higher plants.