Fig. 4

Effect of CpG methylation on minor groove width (MGW) of adjacent A-tracts. a MGW values at the central nucleotide of 3-bp A-tracts, which are shown from AAACG to TTTCG with an exchange of one bp (A/T to T/A) from the 3′ end. Methylation did not decrease MGW at the central bp, except in the ATTCG sequence. Wilcoxon test P values were calculated for methylation narrowing the minor groove at the central nucleotide as the alternative hypothesis (*0.01 < P value ≤ 0.05; **0.001 < P value ≤ 0.01). Four A-tracts followed by a CpG step at the 3′ end include A-tracts preceded by a CpG step at the 5′ end because of symmetry in sequence and cytosine methylation. b MGW at the central nucleotide of 4-bp A-tracts follows a bell-shaped curve from AAAAC to TTTTC. One bp at a time was exchanged from A/T to T/A, starting at the 3′ end. Paired t test P values were calculated for methylation narrowing the minor groove at the central bp as the alternative hypothesis. Two pentamers, AATTC and ATTTC, showed significant P values, meaning that methylation narrowed the minor groove. MC simulations were performed on longer DNA fragments containing hexamer sequences with a CpG/mpg bp step at position 5, and MGW values were measured at the central position 3