Polyketide synthase producing 4-hydroxybenzalacetone. Can use p-coumaryl-CoA as substrate but does not accept hexanoyl-CoA, isobutyryl-CoA, isovaleryl-CoA, and acetyl-CoA as a substrates. Catalyzes the initial key reaction step in the biosynthesis of phenylbutanoids. 4-coumaroyl-CoA + H(+) + H2O + malonyl-CoA = 4-hydroxybenzalacetone + 2 CO2 + 2 CoA kcat is 1.79 min(-1) with 4-coumaroyl-CoA as substrate (at pH 8 and 30 degrees Celsius). Optimum pH is 8.0-8.8. Secondary metabolite biosynthesis; flavonoid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.