THE DEVELOPMENTAL EXPRESSION OF LEISHMANIA-DONOVANI A2 AMASTIGOTE-SPECIFIC GENES IS POSTTRANSCRIPTIONALLY MEDIATED AND INVOLVES ELEMENTS LOCATED IN THE 3'-UNTRANSLATED REGION

Leishmania donovani is a protozoan parasite that exists as a free-livi ng promastigote in the sandfly insect vector and as an amastigote insi de the mammalian host macrophage phagolysosome compartment. The L. don ovani A2 genes have been described previously as developmentally expre ssed in amastigotes but can be induced experimentally in promastigotes by a combination of pH and temperature shifts, conditions that mimic the phagolysosomal compartment of the macrophage cell. Considering the importance of the amastigote stage in human infections, we have exami ned the molecular basis for amastigote stage-specific gene expression. Our results provide evidence that A2 developmental expression during the promastigote-to-amastigote cytodiffer entiation is mediated throug h differential RNA stability and involves the A2 mRNA 3'-untranslated region. The site of processing in the S' untranslated region was a maj or factor for the accumulation of A2 mRNAs in cells incubated under ph agolysosomal conditions. The stability of reporter gene transcripts be aring the A2 8'-untranslated region was increased in cells incubated a t low pH, further confirming the importance of pH shift as an inducer for A2 expression. These observations contribute to defining the mecha nism of amastigote-specific gene regulation in L. donovani. We also de monstrate the feasibility of using the A2 locus to express heterologou s genes differentially in the amastigote form of the L. donovani paras ite.