The Stage-regulated Expression ofLeishmania mexicanaCPB Cysteine Proteases Is Mediated by an Intercistronic Sequence Element
The tandemly arrangedCPBgenes ofLeishmania mexicanaare polycistronically transcribed and encode cysteine proteases that are differentially stage-specific;CPB1andCPB2are expressed predominantly in metacyclics, whereasCPB3–CPB18are expressed mainly in amastigotes. The mechanisms responsible for this differential expression have been studied via gene analysis and re-integration of individualCPBgenes, and variants thereof, into aCPB-deficient parasite mutant. Comparison of the nucleotide sequences of the repeat units ofCPB1andCPB2withCPB2.8(typical ofCPB3–CPB18) revealed two major regions of divergence as follows: one of 258 base pairs (bp) corresponding to the C-terminal extension of CPB2.8; another, designated InS, of 120 bp, with insertions totaling 57 bp, localized to the intercistronic region downstream ofCPB1andCPB2. Cell lines expressingCPB2.8orCPB2with the 3′-untranslated region and intercistronic sequence ofCPB2.8showed up-regulation in amastigotes. Conversely, metacyclic-specific expression occurred withCPB2orCPB2.8with the 3′-untranslated region and intercistronic sequence ofCPB2. Moreover, the InS down-regulated expression in amastigotes of a reporter gene integrated into theCPBlocus. It is proposed that the InS mediates metacyclic-specific stage-regulated expression ofCPBby affecting the maturation of polycistronic pre-mRNA. This is the first well definedcis-regulatory element implicated in post-transcriptional stage-specific gene expression inLeishmania.