<P><B>Abstract</B><P>Highly active mutant of NADPH-dependent acetoacetyl-CoA reductase (PhaB) was expressed in Nicotiana tabacum cv. Bright Yellow-2 cultured cells to produce poly(3-hydroxybutyrate) [P(3HB)]. The mutated PhaB increased P(3HB) content by three-fold over the control, indicating that the mutant was a versatile tool for P(3HB) production. Additionally, the PhaB-catalyzed reaction was suggested to be a rate-limiting step of P(3HB) biosynthesis in tobacco BY-2 cells.</P></P>