Genetic and Biochemical Characterizations of aLhr1 Helicase in the Thermophilic Crenarchaeon Sulfolobus acidocaldarius

Homologous recombination (HR) refers to the process of information exchange between homologous DNA duplexes and is composed of four main steps: end resection, strand invasion and formation of a Holliday junction (HJ), branch migration, and resolution of the HJ. Within each step of HR in Archaea, the helicase-promoting branch migration is not fully understood. Previous biochemical studies identified three candidates for archaeal helicase promoting branch migration in vitro: Hjm/Hel308, PINA, and archaeal long helicase related (aLhr) 2. However, there is no direct evidence of their involvement in HR in vivo. Here, we identified a novel helicase encoded by Saci_0814, isolated from the thermophilic crenarchaeon Sulfolobus acidocaldarius; the helicase dissociated a synthetic HJ. Notably, HR frequency in the Saci_0814-deleted strain was lower than that of the parent strain (5-fold decrease), indicating that Saci_0814 may be involved in HR in vivo. Saci_0814 is classified as an aLhr1 under superfamily 2 helicases; its homologs are conserved among Archaea. Purified protein produced in Escherichia coli showed branch migration activity in vitro. Based on both genetic and biochemical evidence, we suggest that aLhr1 is involved in HR and may function as a branch migration helicase in S. acidocaldarius.

Polyamine–Oligonucleotide Conjugates: 2′-OMe-Triazole-Linked 1,4,7,10-Tetraazacyclododecane and Intercalating Dyes and Their Effect on the Thermal Stability of DNA Duplexes

Oligonucleotides with the sequences 5′-GTG AUPA TGC, 5′-GCA TAUP CAC and 5′-GUPG ATA UPGC, where UP is 2′-O-propargyl uridine, were subjected to post-synthetic Cu(I)-catalyzed azide–alkyne cycloaddition to attach 1,4,7,10-tetraazacyclododecane (cyclen) and two well-known DNA intercalating dyes: thioxanthone and 1,8-naphthalimide. We propose a convenient cyclen protection–deprotection strategy that allows efficient separation of the resulting polyamine–oligonucleotide conjugates from the starting materials by RP-HPLC to obtain high-purity products. In this paper, we present hitherto unknown macrocyclic polyamine–oligonucleotide conjugates and their hybridization properties reflected in the thermal stability of thirty-two DNA duplexes containing combinations of labeled strands, their unmodified complementary strands, and strands with single base pair mismatches. Circular dichroism measurements showed that the B-conformation is retained for all dsDNAs consisting of unmodified and modified oligonucleotides. An additive and destabilizing effect of cyclen moieties attached to dsDNAs was observed. Tm measurements indicate that placing the hydrophobic dye opposite to the cyclen moiety can reduce its destabilizing effect and increase the thermal stability of the duplex. Interestingly, the cyclen-modified U showed significant selectivity for TT mismatch, which resulted in stabilization of the duplex. We conclude the paper with a brief review and discussion in which we compare our results with several examples of oligonucleotides labeled with polyamines at internal strand positions known in the literature.

RPA phosphorylation facilitates RAD52 dependent homologous recombination in BRCA-deficient cells

Loss of RAD52 is synthetically lethal in BRCA-deficient cells, owing to its role in backup homologous recombination (HR) repair of DNA double-strand breaks (DSBs). In HR in mammalian cells, DSBs are processed to single-stranded DNA (ssDNA) overhangs, which are then bound by Replication Protein A(RPA). RPA is exchanged for RAD51 by mediator proteins: in mammals BRCA2 is the primary mediator, however, RAD52 provides an alternative mediator pathway in BRCA-deficient cells. RAD51 stimulates strand exchange between homologous DNA duplexes, a critical step in HR. RPA phosphorylation and de-phosphorylation are important for HR, but its effect on RAD52 mediator function is unknown. Here, we show that RPA phosphorylation is required for RAD52 to salvage HR in BRCA-deficient cells. Using BRCA2-depleted human cells, in which the only available mediator pathway is RAD52-dependent, the expression of phosphorylation-deficient RPA mutant reduced HR. Furthermore, RPA-phospho-mutant cells showed reduced association of RAD52 with RAD51. Interestingly, there was no effect of RPA phosphorylation on RAD52 recruitment to repair foci. Finally, we show that RPA phosphorylation does not affect RAD52-dependent ssDNA annealing. Thus, although RAD52 can be recruited independently of RPA’s phosphorylation status, RPA phosphorylation is required for RAD52’s association with RAD51, and its subsequent promotion of RAD52-mediated HR.

Rapid assessment of Watson–Crick to Hoogsteen exchange in unlabeled DNA duplexes using high-power SELOPE imino ¹H CEST

In duplex DNA, Watson–Crick A–T and G–C base pairs (bp's) exist in dynamic equilibrium with an alternative Hoogsteen conformation, which is low in abundance and short-lived. Measuring how the Hoogsteen dynamics varies across different DNA sequences, structural contexts and physiological conditions is key for identifying potential Hoogsteen hot spots and for understanding the potential roles of Hoogsteen base pairs in DNA recognition and repair. However, such studies are hampered by the need to prepare 13C or 15N isotopically enriched DNA samples for NMR relaxation dispersion (RD) experiments. Here, using SELective Optimized Proton Experiments (SELOPE) 1H CEST experiments employing high-power radiofrequency fields (B1 > 250 Hz) targeting imino protons, we demonstrate accurate and robust characterization of Watson–Crick to Hoogsteen exchange, without the need for isotopic enrichment of the DNA. For 13 residues in three DNA duplexes under different temperature and pH conditions, the exchange parameters deduced from high-power imino 1H CEST were in very good agreement with counterparts measured using off-resonance 13C / 15N spin relaxation in the rotating frame (R1ρ). It is shown that 1H–1H NOE effects which typically introduce artifacts in 1H-based measurements of chemical exchange can be effectively suppressed by selective excitation, provided that the relaxation delay is short (≤ 100 ms). The 1H CEST experiment can be performed with ∼ 10× higher throughput and ∼ 100× lower cost relative to 13C / 15N R1ρ and enabled Hoogsteen chemical exchange measurements undetectable by R1ρ. The results reveal an increased propensity to form Hoogsteen bp's near terminal ends and a diminished propensity within A-tract motifs. The 1H CEST experiment provides a basis for rapidly screening Hoogsteen breathing in duplex DNA, enabling identification of unusual motifs for more in-depth characterization.

Rapid measurement of Watson-Crick to Hoogsteen exchange in unlabeled DNA duplexes using high-power SELOPE imino ¹H CEST

In duplex DNA, Watson-Crick A-T and G-C base pairs (bps) exist in dynamic equilibrium with an alternative Hoogsteen conformation, which is low in abundance and short-lived. Measuring how the Hoogsteen dynamics varies across different DNA sequences, structural contexts and physiological conditions is key for understanding the role of these non-canonical bps in DNA recognition and repair. However, such studies are hampered by the need to prepare 13C or 15N isotopically enriched DNA samples for NMR relaxation dispersion (RD) experiments. Here, using SELective Optimized Proton Experiments (SELOPE) 1H CEST experiments employing high-power radiofrequency fields (B1 > 250 Hz) targeting imino protons, we demonstrate accurate and robust characterization of Waston-Crick to Hoogsteen exchange, without the need for isotopic enrichment of the DNA. For 13 residues in three DNA duplexes under different temperature and pH conditions, the exchange parameters deduced from high-power imino 1H CEST were in very good agreement with counterparts measured using off-resonance 13C/15N spin relaxation in the rotating frame (R1ρ). It is shown that 1H-1H NOE effects which typically introduce artifacts in 1H based measurements of chemical exchange can be effectively suppressed by selective excitation, provided that the relaxation delay is short (≤ 100 ms). The 1H CEST experiment can be performed with ~10X higher throughput and ~100X lower cost relative to 13C/15N R1ρ, and enabled Hoogsteen chemical exchange measurements undetectable by R1ρ. The results reveal an increased propensity to form Hoogsteen bps near terminal ends and a diminished propensity within A-tract motifs. The 1H CEST experiment opens the door to more comprehensively characterizing Hoogsteen breathing in duplex DNA.

Cooperative DNA base and shape recognition by the CCAAT-binding complex and its bZIP transcription factor HapX

The heterotrimeric CCAAT-binding complex (CBC) is a master regulator of transcription. It specifically recognizes the CCAAT-box, a fundamental eukaryotic promoter element. Certain fungi, like Aspergilli, encode a fourth CBC-subunit, HapX, to fine-tune expression of genes involved in iron metabolism. Although being a basic region leucine zipper with its own DNA recognition motif, HapX function strictly relies on the CBC. We here report two crystal structures of the CBC-HapX complex bound to DNA duplexes with distinct sequence and position of HapX sites. In either structure, a HapX dimer targets the nucleic acid downstream of the CCAAT-box and the leash like N-terminus of the distal HapX subunit interacts with CBC and DNA. In vitro and in vivo analyses of HapX mutants support the structures, highlight the complex as an exceptional major and minor groove DNA binder, and enrich our understanding of the functional as well as structural plasticity of related complexes across species.