CDK-Dependent Phosphorylation Regulates PNKP Function in DNA Replication.

Okazaki fragment maturation (OFM) stands as a pivotal DNA metabolic process, crucial for genome integrity and cell viability. Dysregulation of OFM leads to DNA single-strand breaks- accumulation, which is linked to various human diseases such as cancer and neurodegenerative disorders. Recent studies have implicated LIG3-XRCC1 acting in an alternative OFM pathway to the canonical FEN1-LIG1 pathway. Here, we reveal that polynucleotide kinase-phosphatase (PNKP) is another key participant in DNA replication, akin to LIG3-XRCC1. Through functional experiments, we demonstrate PNKP's enrichment at DNA replication forks and its association with PCNA, indicating its involvement in replication processes. Cellular depletion of PNKP mirrors defects observed in OFM-related proteins, highlighting its significance in replication fork dynamics. Additionally, we identify PNKP as a substrate for cyclin-dependent kinase 1/2 (CDK1/2), which phosphorylates PNKP at multiple residues. Mutation analysis of these phosphorylation sites underscores the importance of CDK2-mediated PNKP phosphorylation in DNA replication. Our findings collectively indicate a novel role for PNKP in facilitating Okazaki fragment joining, thus shedding light on its contribution to genome stability maintenance.

The role of RNF138 in DNA end resection is regulated by ubiquitylation and CDK phosphorylation.

Double-strand breaks (DSBs) are DNA lesions that pose a significant threat to genomic stability. The repair of DSBs by the homologous recombination (HR) pathway is preceded by DNA end resection, the 5' to 3' nucleolytic degradation of DNA away from the DSB. We and others previously identified a role for RNF138, a really interesting new gene finger E3 ubiquitin ligase, in stimulating DNA end resection and HR. Yet, little is known about how RNF138's function is regulated in the context of DSB repair. Here, we show that RNF138 is phosphorylated at residue T27 by cyclin-dependent kinase (CDK) activity during the S and G2 phases of the cell cycle. We also observe that RNF138 is ubiquitylated constitutively, with ubiquitylation occurring in part on residue K158 and rising during the S/G2 phases. Interestingly, RNF138 ubiquitylation decreases upon genotoxic stress. By mutating RNF138 at residues T27, K158, and the previously identified S124 ataxia telangiectasia mutated phosphorylation site (Han et al., 2016, ref. 22), we find that post-translational modifications at all three positions mediate DSB repair. Cells expressing the T27A, K158R, and S124A variants of RNF138 are impaired in DNA end resection, HR activity, and are more sensitive to ionizing radiation compared to those expressing wildtype RNF138. Our findings shed more light on how RNF138 activity is controlled by the cell during HR.

Clinical and epidemiological features of the genus Malassezia in Iran.

BACKGROUND AND OBJECTIVES: The genus Malassezia contains an expanding list of lipophilic yeasts involve in the etiology of various superficial fungal infections. Pityriasis versicolor (PV) is the most prevalent Malassezia-related infection distributed worldwide. In the present study, clinical and epidemiological features of the genus Malassezia are discussed with special focus on PV in Iran. MATERIALS AND METHODS: During June 2012 to April 2013, among 713 confirmed cases of fungal infections, 68 (9.5%) were diagnosed as PV by positive direct microscopy results in 20% potassium hydroxide (KOH) preparation of skin scrapings. All the specimens were cultured on modified Dixon agar and incubated at 32°C for 10 days. Identification of the isolated yeasts was carried out based on macro- and microscopic morphology, catalase test, utilization of Tweens, polyethoxylated castor oil (EL slant), and hydrolysis of esculin and utilization of Tween-60 (TE slant). RESULTS: Out of 68 skin scrapings, 55 (80.9%) yielded yeast colonies on mDixon's agar which were finally identified as M. globosa (36.36%), M. pachydermatis (29.08%), M. furfur (23.65%), M. slooffiae (7.28%) and M. obtusa (3.64%). CONCLUSION: Results of the present study further indicate clinico-epidemiological importance of the genus Malassezia with growing importance of M. pachydermatis as a major species involve in the etiology of pityriasis versicolor. These findings are of major concern in management of Malassezia-related diseases.