Polymerizes D(-)-3-hydroxybutyryl-CoA to create polyhydroxybutyrate (PHB) which consists of thousands of hydroxybutyrate molecules linked end to end. This subunit has no catalytic activity but enhances the activity of PhaC, the catalytic subunit. Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Forms a heterodimer with PhaC, which may multimerize in the presence of 3-hydroxybutyryl-CoA. Poly(3-hydroxyalkanoic acids) (PHA), of which PHB is among the most common compounds, has potential uses as a renewable, biodegradable thermoplastic. PHB serves as an intracellular energy reserve material when cells grow under conditions of nutrient limitation. Belongs to the PHA/PHB synthase family. Type III PhaE subfamily.