This present patent application relates to a method for controlling a targeted gene expression with an environmental trigger useful for manufacturing a natural product or an analogue thereof biologically. In particular, the present invention discloses an expression system comprises a gene expressing for a fusion protein of elastin-like polypeptides (ELPs) and a transcription factor, wherein a targeted gene expression is regulated by the phase change of the fusion protein initiated by an environmental trigger, including changes of temperature, pH value, ionic strength or a combination thereof. The method, the expression system and their products are within the scope of this invention.