Ribonucleotide reductase (RNR) provides the precursors for the generation of dNTPs, which are required for DNA synthesis and repair. Here, we investigated the function of the major RNR subunits Rnr1 and Rnr3 in telomere elongation in budding yeast. We show that Rnr1 is essential for the sustained elongation of short telomeres by telomerase. In the absence of Rnr1, cells harbor very short, but functional, telomeres, which cannot become elongated by increased telomerase activity or by tethering of telomerase to telomeres. Furthermore, we demonstrate that Rnr1 function is critical to prevent an early onset of replicative senescence and premature survivor formation in telomerase-negative cells but dispensable for telomere elongation by Homology-Directed-Repair. Our results suggest that telomerase has a "basal activity" mode that is sufficient to compensate for the “end-replication-problem” and does not require the presence of Rnr1 and a different "sustained activity" mode necessary for the elongation of short telomeres, which requires an upregulation of dNTP levels and dGTP ratios specifically through Rnr1 function. By analyzing telomere length and dNTP levels in different mutants showing changes in RNR complex composition and activity we provide evidence that the Mec1ATR checkpoint protein promotes telomere elongation by increasing both dNTP levels and dGTP ratios through Rnr1 upregulation in a mechanism that cannot be replaced by its homolog Rnr3.

Homologous recombination (HR) is a mechanism that repairs a variety of DNA lesions. Under certain circumstances, however, HR can generate intermediates that can interfere with other cellular processes such as DNA transcription or replication. Cells have therefore developed pathways that abolish undesirable HR intermediates. The Saccharomyces cerevisiae yeast Srs2 helicase has a major role in one of these pathways. Srs2 also works during DNA replication and interacts with the clamp PCNA. The relative importance of Srs2’s helicase activity, Rad51 removal function, and PCNA interaction in genome stability remains unclear. We created a new SRS2 allele [srs2(1-850)] that lacks the whole C terminus, containing the interaction site for Rad51 and PCNA and interactions with many other proteins. Thus, the new allele encodes an Srs2 protein bearing only the activity of the DNA helicase. We find that the interactions of Srs2 with Rad51 and PCNA are dispensable for the main role of Srs2 in the repair of DNA damage in vegetative cells and for proper completion of meiosis. On the other hand, it has been shown that in cells impaired for the DNA damage tolerance (DDT) pathways, Srs2 generates toxic intermediates that lead to DNA damage sensitivity; we show that this negative Srs2 activity requires the C terminus of Srs2. Dissection of the genetic interactions of the srs2(1-850) allele suggest a role for Srs2’s helicase activity in sister chromatid cohesion. Our results also indicate that Srs2’s function becomes more central in diploid cells.

Objectives: There is controversy on the risk of neonatal morbidity of sFGR twins. It is plausible that FGR is associated with adverse neonatal outcome, but some studies showed favorable outcome of FGR compared to non-FGR co-twins. The purpose of this study was to compare the risk of neonatal morbidity between sFGR and non-FGR with severe birth weight discordancy (BWD, > 25%) according to chorionicity. Methods: The study population consisted of twins delivered in Seoul National University Hospital between 2001 and 2019. Cases with major congenital anomalies, intrauterine fetal demise, or twin specific complications such as twin-to-twin transfusion syndrome were excluded. Neonatal composite morbidity was defined as the presence of at least one of the followings: respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD), intraventricular hemorrhage (IVH), necrotising enterocolitis (NEC), sepsis, and pneumonia. The risk of neonatal morbidity and mortality were compared between sFGR and non-FGR twins using McNemar test. Results: During the study period, total of 3,677 twins were delivered in our institution. Among them, 242 twins were selected, including 48 pairs of monochorionic (MC) twins and 193 pairs of dichorionic (DC) twins. sFGR twins were at increased risk for neonatal composite morbidity compared with non-FGR twins (16.1% vs 10.7%, p = 0.006). In subgroup analysis, sFGR twins had significantly higher risk for neonatal composite morbidity in DC twins. However, this difference did not reach statistical significance in MC twins. Conclusions: This study shows that sFGR twins complicated with BWD more than 25% had worse neonatal prognosis compared to non-FGR co-twins.Objectives: To compare among birthweight (BW) < 10 th percentile for gestational age, (BW) 10-50 th percentile and (BW) > 50 th percentile; reference group, the strong association of maternal serum markers and the BW degree classifications in the twin pregnancy.