A REGULATORY ROLE FOR THE 5' AND 3' UNTRANSLATED REGIONS IN DIFFERENTIAL EXPRESSION OF HSP83 IN LEISHMANIA

Exposure of Leishmania promastigotes to temperatures typical of mammals results in a stress response, which is accompanied by an increase in the steady state level of heat shock transcripts and their translation. Accumulation of the heat shock protein (hsp83) mRNA occurs due to differential decay rates at the altered temperatures, while transcription is unaffected. A similar pattern of post-transcriptional regulation was observed for a transfected chloramphenicol acetyltransferase (CAT) gene, which was flanked at both ends by intergenic regions (IR) of hsp83. Shortening the 5' untranslated region (UTR) by 100 nts produced an active CAT enzyme, but abolished the temperature-dependent regulation of the CAT - hsp83 mRNA turn-over. The 3' UTR is also involved in the temperature-dependent degradation of hsp83 mRNA, since exchange of the hsp83 3' UTR with a parallel fragment from a non-heat shock gene abolished the differential turn-over of CAT mRNA. Thus, the regulated decay of hsp83 mRNA is controlled by sequence or conformational elements present in both upstream and downstream UTRs. Like the endogenous hsp83, translation of CAT mRNA which contained hsp83 UTRs was higher at 35 degrees C. This was observed only with transcripts in which stability increased at elevated temperatures. Modifications which abolished the temperature dependence of CAT mRNA decay, eliminated its elevated translation at the higher temperatures. The correlation suggests a mechanistic link between the translational machinery and mRNA stability.