When expressed in E.coli with Synechocystis PhaC and C.necator PhaA and PhaB, confers the ability to synthesize up to 13% (w/w) poly(3-hydroxybutyrate) (PHB) depending on the carbon source; all 4 genes are necessary for PHB production (PubMed:9683655). Cell-free in vitro coexpression with PhaE gives a heterodimer able to polymerize 3-hydroxybutyrate-CoA (PubMed:25629766). This subunit has no catalytic activity but enhances the activity of PhaC, the catalytic subunit (By similarity). Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Forms a heterodimer with PhaC, which may multimerize in the presence of 3-hydroxybutyryl-CoA (PubMed:25629766). Both subunits are required for PHB synthesis in E.coli and in PHA-negative A.eutrophus. Poly(3-hydroxyalkanoic acids) (PHA), of which PHB is among the most common compounds, are prokaryotic intracellular storage compounds with potential uses as renewable, biodegradable thermoplastics. Cyanobacterial PHB synthesis is particularly attractive as cyanobacteria use CO(2) as the carbon source. Nitrogen-free medium induces chlorosis in Synechocystis, leading to the degradation of the photosynthetic apparatus and concomitant accumulation of cytoplasmic polyhydroxyalkanoic acid (PHA) granules which in this cyanobacterium are composed of PHB. Belongs to the PHA/PHB synthase family. Type III PhaE subfamily.