NOVELTY - A recombinant microbial host cell comprising at least one DNA molecule encoding polypeptide, is new. The polypeptide catalyzes substrate to product conversion chosen from (a) pyruvate to acetolactate (pathway step a), (b) acetolactate to 2,3-dihydroxy-isovalerate (pathway step b), (c) 2,3-dihydroxy-isovalerate to alpha -ketoisovalerate (pathway step c), (d) alpha -ketoisovalerate to isobutyraldehyde (pathway step d), and (e) isobutyraldehyde to isobutanol (pathway step e).    USE - The microbial host cell chosen from bacterium, Cyanobacterium, filamentous fungus and yeast, is useful for producing isobutanol (claimed), which is used as fuel additive, feedstock and food-grade extractant, and in food and flavor industry and plastic industry.    ADVANTAGE - The microbial host cell is environmentally-friendly and suitable for industrial production, and produces isobutanol inexpensively.    DETAILED DESCRIPTION - A recombinant microbial host cell comprising at least one DNA molecule encoding polypeptide, is new. The polypeptide catalyzes substrate to product conversion chosen from (a) pyruvate to acetolactate (pathway step a), (b) acetolactate to 2,3-dihydroxy-isovalerate (pathway step b), (c) 2,3-dihydroxy-isovalerate to alpha -ketoisovalerate (pathway step c), (d) alpha -ketoisovalerate to isobutyraldehyde (pathway step d), (e) isobutyraldehyde to isobutanol (pathway step e), (f) alpha -ketoisovalerate to isobutyryl-CoA (pathway step f), (g) isobutyryl-CoA to isobutyraldehyde (pathway step g), (h) alpha -ketoisovalerate to valine (pathway step h), (i) valine to isobutylamine (pathway step i) and (j) isobutylamine to isobutyraldehyde (pathway step j). The DNA molecule is heterologous to the microbial host cell. INDEPENDENT CLAIMS are included for the following:    (1) method for producing isobutanol, involves contacting the host cell with a fermentable carbon substrate in a fermentation medium under suitable conditions; and    (2) fermentation medium comprising isobutanol produced by the above-mentioned method.