The moonlighting of RAD23 in DNA repair and protein degradation

The transcription factor PbbHLH164 is destabilized by PbRAD23C/D.1 and mediates ethylene biosynthesis during pear fruit ripening

The phytohormone ethylene plays an important role in climacteric fruit ripening. However, the knowledge on molecular regulation of ethylene biosynthesis remains limited in pear fruit. Herein, a new basic helix-loop-helix transcription factor, PbbHLH164, was identified based on the transcriptome analysis of different developing and ripening fruits of two pear cultivars 'Sucui No. 1' and 'Cuiguan'. PbbHLH164 was more highly expressed in ripening fruit than in developing fruit and positively correlated with ethylene production in both cultivars. PbbHLH164 could directly bind to the promoter of 1-aminocyclopropane-1-carboxylate synthase, PbACS1b, to enhance the expression, leading to the increase of ethylene production and the acceleration of fruit ripening. Interestingly, PbbHLH164 physically interacted with an ubiquitin-like/ubiquitin-associated protein PbRAD23C/D.1, and the interaction of PbbHLH164 with PbRAD23C/D.1 attenuated the function of PbbHLH164 in enhancing the activity of the PbACS1b promoter. Notably, PbRAD23C/D.1 was involved in the degradation of PbbHLH164, and this degradation was inhibited by an ubiquitin proteasome inhibitor MG132. Different from PbbHLH164, PbRAD23C/D.1 was more highly expressed in developing fruit than in ripening fruit of both cultivars. These results suggest that the increase of ethylene production during pear fruit ripening results from the up-regulated expression of PbbHLH164 and the down-regulated expression of PbRAD23C/D.1. This information provided new insights into the molecular regulation of ethylene biosynthesis during fruit ripening.

A Novel Interaction Between RAD23A/B and Y-family DNA Polymerases

The Y-family DNA polymerases - Pol ι, Pol η, Pol κ and Rev1 - are most well-known for their roles in the DNA damage tolerance pathway of translesion synthesis (TLS). They function to overcome replication barriers by bypassing DNA damage lesions that cannot be normally replicated, allowing replication forks to continue without stalling. In this work, we demonstrate a novel interaction between each Y-family polymerase and the nucleotide excision repair (NER) proteins, RAD23A and RAD23B. We initially focus on the interaction between RAD23A and Pol ι, and through a series of biochemical, cell-based, and structural assays, find that the RAD23A ubiquitin-binding domains (UBA1 and UBA2) interact with separate sites within the Pol ι catalytic domain. While this interaction involves the ubiquitin-binding cleft of UBA2, Pol ι interacts with a distinct surface on UBA1. We further find that mutating or deleting either UBA domain disrupts the RAD23A-Pol ι interaction, demonstrating that both interactions are necessary for stable binding. We also provide evidence that both RAD23 proteins interact with Pol ι in a similar manner, as well as with each of the Y-family polymerases. These results shed light on the interplay between the different functions of the RAD23 proteins and reveal novel binding partners for the Y-family TLS polymerases.

Methods for Assessment of Nucleotide Excision Repair Efficiency

Nucleotide excision repair (NER) is responsible for removing a wide variety of bulky adducts from DNA, thus contributing to the maintenance of genome stability. The efficiency with which proteins of the NER system recognize and remove bulky adducts depends on many factors and is of great clinical and diagnostic significance. The review examines current concepts of the NER system molecular basis in eukaryotic cells and analyzes methods for the assessment of the NER-mediated DNA repair efficiency both in vitro and ex vivo.