Abstract: The primary object of the present invention is to provide a polymer synthase gene which is derived from mangrove soil metagenome, and the method for producing the useful copolymer by using this polymer synthase. Another object of the present invention is to provide an enoyl-CoA hydratase gene which is derived from Streptomyces sp. CFMR 7, and the method for producing the useful copolymer, P(3HB-co-3HHx) with increasing the composition of 3HHx by the expression of this enoyl-CoA hydratase. In order to achieve these objects, an isolated polynucleotide encoding for a polypeptide with polymer synthase activity comprising an amino acid sequence set forth in SEQ ID NO: 1 or 3, or an amino acid sequence set forth in SEQ ID NO: 1 or 3 wherein one or more amino acids is replaced, deleted or added are provided.