Kothe, U. | bioRxiv Science

bioRxiv2019

Base pairing interactions between substrate RNA and H/ACA guide RNA modulate the kinetics of pseudouridylation, but not the affinity of substrate binding by H/ACA small nucleolar Ribonucleoproteins

H/ACA small nucleolar ribonucleoproteins (snoRNPs) pseudouridylate RNA in eukaryotes and archaea. They target many RNAs site-specifically through base-pairing interactions between H/ACA guide and substrate RNA. Besides ribosomal RNA (rRNA) and small nuclear RNA (snRNA), H/ACA snoRNPs are thought to also modify messenger RNA (mRNA) with potential impacts on gene expression. However, the base-pairing between known target RNAs and H/ACA guide RNAs varies widely in nature, and therefore the rules governing substrate RNA selection are still not fully understood. To provide quantitative insight into substrate RNA recognition, we systematically altered the sequence of a substrate RNA target by the Saccharomyces cerevisiae H/ACA guide RNA snR34. Time courses measuring pseudouridine formation revealed a gradual decrease in the initial velocity of pseudouridylation upon reducing the number of base pairs between substrate and guide RNA. Changing or inserting nucleotides close to the target uridine severely impairs pseudouridine formation. Interestingly, filter binding experiments show that all substrate RNA variants bind to H/ACA snoRNPs with nanomolar affinity. Next, we showed that binding of inactive, near-cognate RNAs to H/ACA snoRNPs does not inhibit their activity for cognate RNAs, presumably because near-cognate RNAs dissociate rapidly. We discuss that the modulation of initial velocities by the base pairing strength might affect the order and efficiency of pseudouridylation in rRNA during ribosome biogenesis. Moreover, the binding of H/ACA snoRNPs to near-cognate RNAs may be a mechanism to search for cognate target sites. Together, our data provide critical information to aid in the prediction of productive H/ACA guide - substrate RNA pairs.

biochemistry↗

bioRxiv2019

Saccharomyces cerevisiae strains display robust phenotypes in the presence of Dyskeratosis congenita mutations in the Cbf5 gene

Dyskeratosis congenita is a rare, congenital disorder affecting the skin, nails and oral mucosa of patients that often progresses to bone marrow failure and an increased predisposition for a variety of carcinomas. Mutations in the human dyskerin gene have been identified as the most prevalent cause of the disease. Dyskerin is a pseudouridine synthase and the catalytic subunit of H/ACA ribonucleoproteins (RNPs) responsible for the modification of uridines to pseudouridine in ribosomal RNA (rRNA), but dyskerin also binds to the telomerase RNA component (TERC). Accordingly, Dyskeratosis congenita mutations have been reported to affect both telomerase function as well as ribosome biogenesis, but the relative contribution of each pathway to the diseases is under debate. As the yeast homolog of dyskerin, Cbf5, does not interact with telomerase RNA, Saccharomyces cerevisiae is an ideal model to identify the selective impact of Dyskeratosis congenita mutations on ribosome biogenesis. Therefore, chromosomal mutations in the yeast homologue of dyskerin, Cbf5, were introduced at positions corresponding to the mutations in human dyskerin that result in Dyskeratosis congenita. To determine if the mutations affect cellular fitness, we screened for growth defects in yeast. Growth curves at different temperatures and yeast spot assays under several stress conditions revealed that the mutations in cbf5 did not impair growth compared to wild type. These findings suggest that in the yeast cell, Dyskeratosis congenita mutations do not significantly affect ribosome biogenesis, and we discuss the implications for understanding the molecular cause of Dyskeratosis congenita.

molecular biology↗