Histone tail modifications control many nuclear processes by dictating the 
dynamic exchange of regulatory proteins on chromatin. Here we report novel 
insights into histone H3 tail structure in complex with the double PHD finger 
(DPF) of the lysine acetyltransferase MOZ/MYST3/KAT6A. In addition to sampling 
H3 and H4 modification status, we show that the DPF cooperates with the MYST 
domain to promote H3K9 and H3K14 acetylation, although not if H3K4 is 
trimethylated. Four crystal structures of an extended DPF alone and in complex 
with unmodified or acetylated forms of the H3 tail reveal the molecular basis of 
crosstalk between H3K4me3 and H3K14ac. We show for the first time that MOZ DPF 
induces α-helical conformation of H3K4-T11, revealing a unique mode of H3 
recognition. The helical structure facilitates sampling of H3K4 methylation 
status, and proffers H3K9 and other residues for modification. Additionally, we 
show that a conserved double glycine hinge flanking the H3 tail helix is 
required for a conformational change enabling docking of H3K14ac with the DPF. 
In summary, our data provide the first observations of extensive helical 
structure in a histone tail, revealing the inherent ability of the H3 tail to 
adopt alternate conformations in complex with chromatin regulators.