Eukaryotic Ribosomal Expansion Segments as Antimicrobial Targets

Diversity in eukaryotic rRNA structure and function offers possibilities of therapeutic targets. Unlike ribosomes of prokaryotes, eukaryotic ribosomes contain species-specific rRNA expansion segments (ESs) with idiosyncratic structures and functions that are essential and specific to some organisms. Here we investigate expansion segment 7 (ES7), one of the largest and most variable expansions of the eukaryotic ribosome. We hypothesize that ES7 of the pathogenic fungi Candida albicans (ES7_CA) could be a prototypic drug target. We show that isolated ES7_CA folds reversibly to a native-like state. We developed a fluorescence displacement assay using an RNA binding fluorescent probe, F-neo. F-neo binds tightly to ES7_CA with a K_d of 2.5 × 10^-9 M but binds weakly to ES7 of humans (ES7_HS) with a K_d estimated to be greater than 7 μM. The fluorescence displacement assay was used to investigate the affinities of a library of peptidic aminosugar conjugates (PAs) for ES7_CA. For conjugates with highest affinities for ES7_CA (NeoRH, NeoFH, and NeoYH), the lowest dose needed to induce mortality in C. albicans (minimum inhibitory concentration, MIC) was determined. PAs with the lowest MIC values were tested for cytotoxicity in HEK293T cells. Molecules with high affinity for ES7_CA in vitro induce mortality in C. albicans but not in HEK293T cells. The results are consistent with the hypothesis that ESs represent useful targets for chemotherapeutics directed against eukaryotic pathogens.