Histones were identically tagged at their N-termini with biotin ligase recognition peptide (BLRP) (de Boer, E. et al. Proc Natl Acad Sci U S A 100, 7480-5 (2003), and Beckett, D. et al. Protein Sci 8, 921-9 (1999)) and expressed together with E. coli biotin ligase in Drosophila S2 cells. Nuclei were prepared as described (McKittrick, E., et al., Proc. Natl. Acad. Sci. USA 101, 1525-1530 (2004).), and treated with micrococcal nuclease (MNase) to digest chromatin to yield mostly mononucleosomes. Chromatin was extracted (input) using a modification of the procedure of Blower, M.D., et al. (Developmental Cell 2, 319-330 (2002)). Biotinylated histone-containing nucleosome particles (pull-down) were purified and DNA was extracted from both input and pull-down fraction for microarray analysis. DNA from input (3 ug) and pull-down (4 ug) was provided to NimbleGen Systems for differential labelling by random priming with Cy3- or Cy5 and hybridization to oligonucleotide arrays. For each experiment, input and pull-down channel signal intensities and scaled log2(ratios) were provided by NimbleGen Systems.
Data processing
Log base 2 (PM_635/PM_532) ratios were bi-weight mean centered
