Cloning of S -Adenosyl- l -Methionine:C-24-Δ-Sterol-Methyltransferase (ERG6) from Leishmania donovani and Characterization of mRNAs in Wild-Type and Amphotericin B-Resistant Promastigotes

The 24-alkylated sterols have been shown previously to be absent in membranes of amphotericin B (AmB)-resistant Leishmania donovani promastigotes, suggesting that the S- adenosyl-l-methionine:C-24-Delta-sterol-methyltransferase (SCMT or ERG6) was not functional or not expressed in AmB-resistant (AmB-R) parasites. From an L. donovani wild-type clone, we cloned two cDNAs with an identical open reading frame encoding a putative SCMT, the enzyme responsible for a first sterol methylation at the C-24 position. The two cDNAs differed by their 3'-untranslated region (3'-UTR) and 5'-UTR sequences. One transcript (A) had a normal structure with a spliced leader and was highly expressed in normal cells but absent in AmB-R cells. The other (B), which did not possess the spliced leader sequence, was weakly expressed in normal cells but strongly expressed in AmB-R cells. As a functional test, ERG6 null mutant Saccharomyces cerevisiae yeasts were transformed using the pYES2.1 TOPO TA expression vector containing the candidate SCMT1/ERG6 coding sequence cloned from L. donovani. The transformed yeasts exhibited C-24 alkylated sterol expression, mainly ergosterol, within their membranes, proving that the isolated cDNA encodes on a SCMT responsible for sterol methylation. In AmB-R L. donovani promastigotes, the absence of the normal transcript (A) and the expression of an abnormal species (B) devoid of a spliced leader could explain the absence of sterol methylation in these cells. Further studies using a homologous system will allow us to draw conclusions about the relationship between SCMT expression and AmB resistance in Leishmania.