NOVELTY - Recombinant obligate anaerobic bacterial acetogen cells comprising (a) at least one heterologous nucleic acid molecule encoding an isoprene synthase polypeptide and (b) at least one heterologous nucleic acid molecule encoding an isopentenyl diphosphate isomerase polypeptide, are new. The nucleic acid molecules of (a) and (b) are operably linked either separately or in an operon to a promoter that is functional in the cells.    USE - The cells are useful for producing isoprene, mevalonate (all claimed) and/or other industrial bio-products which are useful as fuels.    ADVANTAGE - The cells produce higher yields of highly pure isoprene, when grown in substantially oxygen-free culture conditions comprising synthesis gas as energy and/or carbon source, as compared to the cells without the heterologous nucleic acids, in a robust and cost effective manner.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) producing isoprene comprising culturing cells under suitable conditions for the production of isoprene, where the culture conditions are substantially oxygen-free and comprise synthesis gas as energy and/or carbon source;    (2) recombinant obligate anaerobic bacterial acetogen cells (C2) comprising at least one heterologous nucleic acid encoding at least two polypeptides having acetyl-coenzyme-A acetyltransferase, 3-hydroxy-3- methylglutaryl-coenzyme-A (HMG-CoA) reductase or HMG-CoA synthase activities, where the heterologous nucleic acids are operably linked either separately or in an operon to a promoter that is active in the cells, and the cells produce mevalonate when grown in substantially oxygen-free culture conditions comprising synthesis gas as energy and/or carbon source;    (3) producing mevalonate comprising culturing cells (C2) under suitable conditions for the production of mevalonate, where the culture conditions are substantially oxygen-free and comprise synthesis gas as energy and/or carbon source; and    (4) recombinant obligate anaerobic bacterial acetogen cells (C3) capable of producing acetyl-coenzyme (CoA) derived products, comprising at least one heterologous nucleic acid encoding a polypeptide involved in the conversion of acetyl-CoA into a acetyl-CoA derived product in operable combination with a promoter, where the culturing of the cells under substantially oxygen-free culture conditions comprising synthesis gas as energy and/or carbon source provides for the production of the acetyl-CoA derived product.