Rapid Diagnostic Tests (RDTs) for malaria are restricted to a few biomarkers and antibody-mediated detection. However, the expression of commonly used biomarkers varies geographically and the sensibility of immunodetection can be affected by batch-to-batch differences or limited thermal stability. In this study we aimed to overcome these limitations by identifying a potential biomarker and by developing molecular sensors based on aptamer technology. Using gene expression databases, ribosome profiling analysis, and structural modeling, we find that the High Mobility Group Box 1 protein (HMGB1) of
Plasmodium falciparum
is highly expressed, structurally stable, and present along all blood-stages of
P
.
falciparum
infection. To develop biosensors, we used
in vitro
evolution techniques to produce DNA aptamers for the recombinantly expressed HMG-box, the conserved domain of HMGB1. An evolutionary approach for evaluating the dynamics of aptamer populations suggested three predominant aptamer motifs. Representatives of the aptamer families were tested for binding parameters to the HMG-box domain using microscale thermophoresis and rapid kinetics. Dissociation constants of the aptamers varied over two orders of magnitude between nano- and micromolar ranges while the aptamer-HMG-box interaction occurred in a few seconds. The specificity of aptamer binding to the HMG-box of
P
.
falciparum
compared to its human homolog depended on pH conditions. Altogether, our study proposes HMGB1 as a candidate biomarker and a set of sensing aptamers that can be further developed into rapid diagnostic tests for
P
.
falciparum
detection.
Mammalian Dicer is the gatekeeper into the essential gene-regulating miRNA pathway. What is committing mammalian Dicer to the miRNA pathway remains unknown. We report that Dicer's highly conserved DExD/H helicase domain is the key structural element supporting accurate miRNA biogenesis. While ATPase activity of the domain is non-essential, its loss is lethal in mice. It is required during canonical miRNA biogenesis for efficient recognition, high-fidelity cleavage, and strand selection. Structure of Dicer-miRNA precursor complexes showed that the DExD/H domain acquired helicase-unrelated function defining Dicer conformations, which affect substrate loading and facilitate pre-selection of miRNA precursors. Dicer lacking the DExD/H domain favors conformations enabling reduced substrate selectivity and supporting RNA interference, a different small RNA pathway. Therefore, Dicer's DExD/H domain ensures indispensable high-fidelity precursor processing of mammalian miRNAs, which constitutes a structural "mold" for adaptive miRNA evolution.
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