Selective inactivation of SIDER2 retroposon-mediated mRNA decay contributes to stage- and species-specific gene expression in Leishmania.

Despite their high genomic synteny, the Leishmania major and Leishmania infantum species exhibit extensive differences in mRNA expression patterns throughout the parasite's development. Yet, the underlying mechanisms for this species-specific differential gene expression are largely unknown. Here we report that Short Interspersed DEgenerated Retroposons of the SIDER2 subfamily, shown previously to promote rapid mRNA turnover, confer differential regulation of orthologous transcripts resulting in a stage- and species-specific gene expression. We demonstrate that SIDER2-mediated decay of two L. major transcripts encoding a hypothetical protein and an aminomethyltransferase to a similar extent in promastigote and amastigote developmental forms results in a constitutive low expression of the corresponding proteins. In contrast, their L. infantum orthologs are differentially expressed due to the selective inactivation of SIDER2 in intracellular amastigotes. Inactivation of the SIDER2 function blocks the SIDER2-mediated deadenylation-independent decay pathway, and stabilized transcripts are degraded by a slower, deadenylation-dependent mechanism. Sequence variations in SIDER2 retroposons between orthologous transcripts do not contribute to SIDER2 inactivation. Our data suggest that SIDER2 inactivation is 3'-untranslated region context-dependent and that involves possibly species- and stage-specific trans-acting factor(s). These findings further emphasize the important contribution of SIDER retroposons in the control of gene expression across the Leishmania genus.