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<strong>Protein</strong>s Cross-linked to DNA by Formaldehyde 757<br />

A 260 × 50 µg/mL × 100 × (volume of cell lysate)/1000 µg/mg of DNA<br />

One A 260 unit represents 50 mg of DNA/mL of cell lysate. To determine the micrograms<br />

of DNA in 1 mL of cell lysate, multiply the absorbance reading by 50 and then by the<br />

dilution factor (i.e., 100). Multiply the resulting concentration by the total volume of cell<br />

lysate to determine the total mg of DNA in the cell lysate. Divide the total micrograms by<br />

1000 to convert this value into milligrams of DNA. The resulting amount of cellular DNA<br />

is only an approximation, as some proteins within the cell lysate will have a peak absorption<br />

at 260 nm.<br />

9. Gently inverting the hydroxyapatite resin when mixing avoids damaging the integrity of<br />

the resin.<br />

10. The porosity of the dialysis tubing will depend on the size of the protein of interest.<br />

11. Dialysis tubing should be soaked in distilled water for at least 30 min before use.<br />

Acknowledgments<br />

Our research was supported by grants from the Medical Research Council of Canada<br />

(MT-9186, RO-15183), CancerCare Manitoba, U.S. Army Medical and Materiel Command<br />

Breast Cancer Research Program (DAMD17-001-10319), and the National Cancer<br />

Institute of Canada (NCIC) with funds from the Canadian Cancer Society. A<br />

Medical Research Council of Canada Senior Scientist to J. R. D. and a NCIC<br />

Studentship to V. A. S. are gratefully acknowledged.<br />

References<br />

1. Orlando, V. (2000) Mapping chromosomal proteins by in vivo formaldehyde-crosslinkedchromatin<br />

immunoprecipitation. Trends Biochem. Sci. 25, 99–104.<br />

2. Dedon, P. C., Soults, J. A., Allis, C. D., and Gorovsky, M. A. (1991) A simplified formaldehyde<br />

fixation and immunoprecipitation technique for studying protein-DNA interactions.<br />

Analyt. Biochem. 197, 83–90.<br />

3. Orlando, V., Strutt H., and Paro, R. (1997) Analysis of chromatin structure by in vivo<br />

formaldehyde crosslinking. Methods 11, 205–214.<br />

4. Kadosh, D. and Struhl, K. (1998) Targeted recruitment of the Sin3-Rpd3 histone<br />

deacetylase complex generates a highly localized domain of repressed chromatin in vivo.<br />

Mol. Cell. Biol. 18, 5121–5127.

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