Replication Dynamics

Abstract:

Many experiments involving nucleic acids require the hybridization and ligation of multiple DNA or RNA molecules to form a compound molecule. When one of the constituents is single stranded, however, the efficiency of ligation can be very low and requires significant individually tailored optimization. Also, when the molecules involved are very long (>10 kb), the reaction efficiency typically reduces dramatically. Here, we present a simple procedure to efficiently and specifically end-join two different nucleic acids using the well-known biotin-streptavidin linkage. We introduce a two-step approach, in which we initially bind only one molecule to streptavidin (STV). The second molecule is added only after complete removal of the unbound STV. This primarily forms heterodimers and nearly completely suppresses formation of unwanted homodimers. We demonstrate that the joining efficiency is 50 +/- 25% and is insensitive to molecule length (up to at least 20 kb). Furthermore, our method eliminates the requirement for specific complementary overhangs and can therefore be applied to both DNA and RNA. Demonstrated examples of the method include the efficient end-joining of DNA to single-stranded and double-stranded RNA, and the joining of two double-stranded RNA molecules. End-joining of long nucleic acids using this procedure may find applications in bionanotechnology and in single-molecule experiments.

Abstract:

Ligases are essential actors in DNA replication, recombination, and repair by virtue of their ability to seal breaks in the phosphodiester backbone. Ligation proceeds through a nicked DNA-adenylate intermediate (AppDNA), which must be sealed quickly to avoid creating a potentially toxic lesion. Here, we take advantage of ligase-catalyzed AMP-dependent incision of a single supercoiled DNA molecule to observe the step of phosphodiester synthesis in real time. An exponentially distributed number of supercoils was relaxed per successful incision-resealing event, from which we deduce the torque-dependent ligation probability per DNA swivel. Premature dissociation of ligase from nicked DNA-adenylate accounted for approximately 10% of the observed events. The ability of ligase to form a C-shaped protein clamp around DNA is a key determinant of ligation probability per turn and the stability of the ligase-AppDNA intermediate. The estimated rate of phosphodiester synthesis by DNA ligase (400 s(-1)) is similar to the high rates of phosphodiester synthesis by replicative DNA polymerases.

Abstract:

The rate of DNA supercoil removal by human topoisomerase IB (TopIB) is slowed down by the presence of the camptothecin class of antitumor drugs. By preventing religation, these drugs also prolong the lifetime of the covalent TopIB-DNA complex. Here, we use magnetic tweezers to measure the rate of supercoil removal by drug-bound TopIB at a single DNA sequence in real time. This is accomplished by covalently linking camptothecins to a triple helix-forming oligonucleotide that binds at one location on the DNA molecule monitored. Surprisingly, we find that the DNA dynamics with the TopIB-drug interaction restricted to a single DNA sequence are indistinguishable from the dynamics observed when the TopIB-drug interaction takes place at multiple sites. Specifically, the DNA sequence does not affect the instantaneous supercoil removal rate or the degree to which camptothecins increase the lifetime of the covalent complex. Our data suggest that sequence-dependent dynamics need not to be taken into account in efforts to develop novel camptothecins.

Abstract:

We study ionic current fluctuations in solid-state nanopores over a wide frequency range and present a complete description of the noise characteristics. At low frequencies (f approximately < 100 Hz) we observe 1/f-type of noise. We analyze this low-frequency noise at different salt concentrations and find that the noise power remarkably scales linearly with the inverse number of charge carriers, in agreement with Hooge's relation. We find a Hooge parameter alpha = (1.1 +/- 0.1) x 10(-4). In the high-frequency regime (f approximately > 1 kHz), we can model the increase in current power spectral density with frequency through a calculation of the Johnson noise. Finally, we use these results to compute the signal-to-noise ratio for DNA translocation for different salt concentrations and nanopore diameters, yielding the parameters for optimal detection efficiency.

Abstract:

Increasing the ability of chemotherapeutic drugs to kill cancer cells is often hampered by a limited understanding of their mechanism of action. Camptothecins, such as topotecan, induce cell death by poisoning DNA topoisomerase I, an enzyme capable of removing DNA supercoils. Topotecan is thought to stabilize a covalent topoisomerase-DNA complex, rendering it an obstacle to DNA replication forks. Here we use single-molecule nanomanipulation to monitor the dynamics of human topoisomerase I in the presence of topotecan. This allowed us to detect the binding and unbinding of an individual topotecan molecule in real time and to quantify the drug-induced trapping of topoisomerase on DNA. Unexpectedly, our findings also show that topotecan significantly hinders topoisomerase-mediated DNA uncoiling, with a more pronounced effect on the removal of positive (overwound) versus negative supercoils. In vivo experiments in the budding yeast verified the resulting prediction that positive supercoils would accumulate during transcription and replication as a consequence of camptothecin poisoning of topoisomerase I. Positive supercoils, however, were not induced by drug treatment of cells expressing a catalytically active, camptothecin-resistant topoisomerase I mutant. This combination of single-molecule and in vivo data suggests a cytotoxic mechanism for camptothecins, in which the accumulation of positive supercoils ahead of the replication machinery induces potentially lethal DNA lesions.