NOVELTY - Producing butanol or 2-butanone, by contacting recombinant host cell comprising first heterologous polynucleotide comprising nucleic acid sequence encoding biocatalyst polypeptide or biocatalyst polypeptide with nonsense or frameshift allele and second heterologous polynucleotide comprising promoter nucleic acid sequence, and nucleic acid sequence encoding suppressor tRNA or anti-sense RNA with carbon substrate under first set of conditions, and contacting the recombinant host cell with the carbon substrate under second set of conditions.    USE - The method (M1) is useful for producing butanol or 2-butanone. The method (M2) is useful for production of fermentation product chosen from butanol, 2-butanone, propanol, isopropanol and ethanol, preferably butanol or 2-butanone, more preferably isobutanol (all claimed). The butanol is useful as industrial chemicals, fuel additive, feedstock chemical in plastics industry, and food-grade extractant in the food and flavor industry.    ADVANTAGE - The methods are cost-effective and environmentally-friendly as the butanol or 2-butanone is produced using plant-derived raw materials that in turn minimize green house gas emissions, and enable high yield production of the butanol or 2-butanone without production of toxic products, as low sugar concentration is maintained during the production method.    DETAILED DESCRIPTION - Method (M1) for producing butanol or 2-butanone, involves (a) contacting recombinant host cell comprising first heterologous polynucleotide and second heterologous polynucleotide, where the first heterologous polynucleotide comprises nucleic acid sequence encoding biocatalyst polypeptide or biocatalyst polypeptide with nonsense or frameshift allele, and the second heterologous polynucleotide comprises (i) promoter nucleic acid sequence, and (ii) nucleic acid sequence encoding suppressor tRNA specific to a nonsense or frameshift allele of the first heterologous polynucleotide or anti-sense RNA specific to the mRNA encoded by the first heterologous polynucleotide, with carbon substrate under first set of conditions, and (b) contacting the recombinant host cell with the carbon substrate under second set of conditions, where the first set of conditions and second set of conditions differ and the nucleic acid sequence encoding the suppressor tRNA is differentially expressed under the first set of conditions than under the second set of conditions, and the host cell produces butanol or 2-butanone under the first set of conditions and/or second set of conditions. INDEPENDENT CLAIMS are included for the following:    (1) an isolated polynucleotide comprising (a) a promoter nucleic acid sequence, and (b) a nucleic acid sequence encoding a suppressor tRNA or anti-sense RNA, where the nucleic acid sequence of (b) is coupled to the nucleic acid sequence of (a) such that the suppressor tRNA or anti-sense RNA is differentially expressed during the production phase and the propagation phase of a fermentation process;    (2) recombinant microbial host cell comprising the above-mentioned isolated polynucleotide; and    (3) method (M2) for production of fermentation product, involves (a) providing the above-mentioned recombinant host cell, (b) contacting the host cell with fermentable carbon substrate in a fermentation medium under conditions for production of the fermentation product, and (c) recovering the fermentation product.