No abstract

Nucleobase-directed spin-labeling by the azide-alkyne 'click' (CuAAC) reaction has been performed for the first time with oligonucleotides. 7-Deaza-7-ethynyl-2'-deoxyadenosine (1) and 5-ethynyl-2'-deoxyuridine (2) were chosen to incorporate terminal triple bonds into DNA. Oligonucleotides containing 1 or 2 were synthesized on a solid phase and spin labeling with 4-azido-2,2,6,6-tetramethylpiperidine 1-oxyl (4-azido-TEMPO, 3) was performed by post-modification in solution. Two spin labels (3) were incorporated with high efficiency into the DNA duplex at spatially separated positions or into a 'dA-dT' base pair. Modification at the 5-position of the pyrimidine base or at the 7-position of the 7-deazapurine residue gave steric freedom to the spin label in the major groove of duplex DNA. By applying cw and pulse EPR spectroscopy, very accurate distances between spin labels, within the range of 1-2 nm, were measured. The spin-spin distance was 1.8±0.2 nm for DNA duplex 17(dA*(7,10))⋅11 containing two spin labels that are separated by two nucleotides within one individual strand. A distance of 1.4±0.2 nm was found for the spin-labeled 'dA-dT' base pair 15(dA*(7))⋅16(dT*(6)). The 'click' approach has the potential to be applied to all four constituents of DNA, which indicates the universal applicability of the method. New insights into the structural changes of canonical or modified DNA are expected to provide additional information on novel DNA structures, protein interaction, DNA architecture, and synthetic biology.

The assembly of prereplicative complex (pre-RC) during G1 phase must be tightly controlled to sustain cell proliferation and maintain genomic stability. Mechanisms to prevent pre-RC formation in G2/M and S phases are well appreciated, whereas how cells ensure efficient pre-RC assembly during G1 is less clear. Here we report that cyclin K regulates pre-RC formation. We find that cyclin K expression positively correlates with cell proliferation, and knockdown of cyclin K or its cognate kinase CDK12 prevents the assembly of pre-RC in G1 phase. Mechanistically we uncover that cyclin K promotes pre-RC assembly by restricting cyclin E1 activity in G1. We identify a cyclin K-dependent, novel phosphorylation site in cyclin E1 that disrupts its interaction with CDK2. Importantly, this antagonistic relationship is largely recapitulated in cyclin E1-overexpressing tumors. We discuss the implications of our findings in light of recent reports linking cyclin K and CDK12 to human tumorigenesis.