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8‑Oxo-7,8-dihydroguanine in the Context of a Gene Promoter G‑Quadruplex Is an On–Off Switch for Transcription
journal contribution
posted on 2017-08-22, 00:00 authored by Aaron M. Fleming, Judy Zhu, Yun Ding, Cynthia J. BurrowsInterplay
between DNA repair of the oxidatively modified base 8-oxo-7,8-dihydroguanine
(OG) and transcriptional activation has been documented in mammalian
genes. Previously, we synthesized OG into the VEGF potential G-quadruplex sequence (PQS) in the coding strand of a
luciferase promoter to identify that base excision repair (BER) unmasked
the G-quadruplex (G4) fold for gene activation. In the present work,
OG was site-specifically synthesized into a luciferase reporter plasmid
to follow the time-dependent expression in mammalian cells when OG
in the VEGF PQS context was located in the coding
vs template strands of the luciferase promoter. Removal of OG from
the coding strand by OG glycosylase-1 (OGG1)-mediated BER upregulated
transcription. When OG was in the template strand in the VEGF PQS context, transcription was downregulated by a BER-independent
process. The time course changes in transcription show that repair
in the template strand was more efficient than repair in the coding
strand. Promoters were synthesized with an OG:A base pair that requires
repair on both strands to yield a canonical G:C base pair. By monitoring
the up/down luciferase expression, we followed the timing of repair
of an OG:A base pair occurring on both strands in mammalian cells
in which one lesion resides in a G-quadruplex loop and one in a potential
i-motif. Depending on the strand in which OG resides, coding vs template,
this modification is an up/downregulator of transcription that couples
DNA repair with transcriptional regulation.