Biosciences Department Seminar
"Chromatin's Resistance to DNA Repair"
Presented by John M. Hinz, School of Molecular Biosciences, Washington State University, Pullman, WA
Friday, March 15, 2013, 11 am
Biosciences Seminar Room, Bldg. 463
Hosted by: Paul Wilson
The human genome is under constant assault by agents that cause mutagenic damage, a key factor in our development of cancer and age-related traits. What's worse, studies are revealing that the many proteins associated with DNA, including histones (essential for DNA compaction and transcriptional regulation), inhibit the DNA repair activities in the cell necessary for recognition and removal of mutagenic lesions. We have examined the ability of the Uracil DNA glycosylase (UDG), responsible for recognition and removal of erroneous uracil residues in DNA, to detect lesions in DNA substrate bound to a histone octamer (also known as a nucleosome core particle). We have found that when the uracil is rotationally positioned such that its nucleotide backbone is facing "inward" towards the histones, the rate of its removal is greatly reduced with respect to uracils positioned at other orientations. Interestingly, reducing movement of the DNA on the surface of the nucleosome core particles (by chemical crosslinking) reduces the rate of UDG activity on the inward facing uracil, but increases the UDG activity on "outward" oriented uracil residues, highlighting a role for DNA-histone dynamics in recognition of DNA lesions by glycosylases. Uracil lesions do not perturb the secondary structure of DNA, but some lesions distort the helix and thus may not be completely hidden from repair factors by histones. Using Förster Resonance Energy Transfer we found that DNA containing damage caused by ultraviolet light (leading to pyrimidine dimer kinks in the helix) has a reduced affinity to histone octamers. Thus, helix-distorting lesions provide an intrinsic means of revealing sites of damage in chromatin and may therefor promote their own repair.