Following prolonged genotoxic stress, DNA2 and WRN functionally interact to degrade reversed replication forks and promote replication restart, thereby preventing aberrant processing of unresolved replication intermediates

Background: Dna2 and FEN1 putatively cooperate in Okazaki fragment (OF) processing in yeast. Results: FEN1 depletion leads to OF maturation defects that do not impact replication fork kinetics, whereas hDna2 depletion leads to DNA damage independent of OF maturation. Conclusion: hDna2 participates in DNA replication in a manner distinct from FEN1. Significance: hDna2 participates in replication in a FEN1, OF maturation-independent manner.

We describe a method for synchronization of the cell cycle in the bacterium E. coli. Treatment of asynchronous cultures with the amino acid analog, DL-serine hydroxamate, induces the stringent response, with concomitant arrest of DNA replication at initiation. Following release of the stringent response, cells initiate DNA replication in synchrony, as determined by flow cytometry for DNA content, Southern blotting and microscopy. This method has the advantage that it can be used in fully wild-type cells, at different growth rates, and may be applicable to other bacterial species with replication control by the stringent response. We also elaborate other methods useful for establishing cell cycle parameters in bacterial populations. We describe flow cytometric methods for analyzing bacterial populations for DNA content using the DNA-specific dye PicoGreen, readily detected by most commercial flow cytometers. We also present an method for incorporation of the nucleotide ethynyl-deoxyuridine, EdU, followed by "click" labeling with fluorescent dyes, which allows us to measure and visualize newly replicated DNA in fixed E. coli K-12 cells under non-denaturing conditions.

SummaryThe stringent response is important for bacterial survival under stressful conditions, such as amino acid starvation, and is characterized by the accumulation of ppGpp and pppGpp. ObgE (CgtA, YhbZ) is an essential conserved GTPase in Escherichia coli and several observations have implicated the protein in the control of the stringent response. However, consequences of the protein on specific responses to amino acid starvation have not been noted. We show that ObgE binds to ppGpp with biologically relevant affinity in vitro, implicating ppGpp as an in vivo ligand of ObgE. ObgE mutants increase the ratio of pppGpp to ppGpp within the cell during the stringent response. These changes are correlated with a delayed inhibition of DNA replication by the stringent response, delayed resumption of DNA replication after release, as well as a decreased survival after amino acid deprivation. With these data, we place ObgE as an active effector of the response to amino acid starvation in vivo. Our data correlate the pppGpp/ppGpp ratio with DNA replication control under bacterial starvation conditions, suggesting a possible role for the relative balance of these two nucleotides.

Background:Telomere fragility occurs following replication stress. Results: Leading strand-specific telomere fragility is induced by FEN1 depletion and transcription inhibition and is rescued by ectopic RNase H1 expression. Conclusion: RNA:DNA hybrids contribute to telomere fragility that is limited by FEN1. Significance: This is the first explanation for leading strand-specific telomere fragility and is the first leading strand-specific role for FEN1.