Coordinated Zac binding regulates in vitro the catalytic activity of p300-HAT. (A) p300 (1 nM) was incubated in the absence or presence of equimolar amounts of Zac, ZacΔC1, or ZacΔZF6 with saturating amounts of H4 peptide and [3H]acetyl-CoA for the indicated times. All reactions contained the DR element. Linear least-squares fit was used to calculate maximal vz. (B and C) vz of p300ΔKIX or of p300ΔCH3 (1 nM each) in the absence or presence of equimolar amounts of Zac was determined as described above. (A through C) Data represent means and standard deviations from at least three experiments done in duplicate. (D) Survey of Km(h), Km(a), and vz values for p300 and derivatives in the absence or presence of Zac, ZacΔC1, and ZacΔZF6.