Identification of exceptionally potent adenosine deaminases RNA editors from high body temperature organisms.

The most abundant form of RNA editing in metazoa is the deamination of adenosines into inosines (A-to-I), catalyzed by ADAR enzymes. Inosines are read as guanosines by the translation machinery, and thus A-to-I may lead to protein recoding. The ability of ADARs to recode at the mRNA level makes them attractive therapeutic tools. Several approaches for Site-Directed RNA Editing (SDRE) are currently under development. A major challenge in this field is achieving high on-target editing efficiency, and thus it is of much interest to identify highly potent ADARs. To address this, we used the baker yeast Saccharomyces cerevisiae as an editing-naïve system. We exogenously expressed a range of heterologous ADARs and identified the hummingbird and primarily mallard-duck ADARs, which evolved at 40-42°C, as two exceptionally potent editors. ADARs bind to double-stranded RNA structures (dsRNAs), which in turn are temperature sensitive. Our results indicate that species evolved to live with higher core body temperatures have developed ADAR enzymes that target weaker dsRNA structures and would therefore be more effective than other ADARs. Further studies may use this approach to isolate additional ADARs with an editing profile of choice to meet specific requirements, thus broadening the applicability of SDRE.

Reversal of histone H2B mono-ubiquitination is required for replication stress recovery.

Mono-ubiquitination of histone H2B (H2B-Ub1) is a conserved modification that plays central role in regulating numerous biological processes including the DNA damage response, gene transcription, and DNA replication. Previous studies have revealed that H2B-Ub1 promotes recovery from replication stress by mediating Rad53 phosphorylation (Rad53-P), and activation of the intra-S replication checkpoint, in order to limit fork progression, and associated DNA damage. Since such mono-ubiquitination is a reversible process, we examined the role of H2B-Ub1 deubiquitination during replication stress. Using an experimental system in yeast which mimics H2B-Ub1 accumulation, we show that cells become sensitive to the replication stress induced by HU. This stress response was accompanied by Rad53-P accumulation, and delayed recovery from intra-S checkpoint arrest. Furthermore, we show that similar effects were recapitulated by the accumulation of endogenous H2B-Ub1, induced by the co-inactivation of the deubiquitinating enzyme, Ubp10, and Spt16, a FACT histone chaperone family member. While it has been well established that H2B mono-ubiquitination plays an essential role in recovering from replication stress, our data reveal that H2B-Ub1 deubiquitination is also essential for this process.