NOVELTY - A recombinant microorganism comprising at least one nucleic acid molecule encoding a ketol-acid reductoisomerase (KARI), is new.    USE - For producing isobutanol (claimed); and for producing beneficial metabolites including fuels and chemicals.    ADVANTAGE - The use of the S. exigua KARI enzyme in an isobutanol pathway leads to improved isobutanol yields, titers, and productivity. The modified or mutated KARI exhibit an increased catalytic efficiency with nicotinamide adenine dinucleotide hydrogenase (NADH), a decreased Michaelis Menten constant for NADH, an increased catalytic constant (k(cat)) with NADH, and an increased Michaelis Menten constant for NADPH, as compared to the wild-type KARL.    DETAILED DESCRIPTION - A recombinant microorganism comprising at least one nucleic acid molecule encoding a ketol-acid reductoisomerase (KARI), where the KARI is at least 60% identical to SEQ ID NO: 2, as available on WIPO website, is new. INDEPENDENT CLAIMS are included for the following: 1) new isolated nucleic acid molecule encoding a ketol-acid reductoisomerase (KARI), where the KARI is at least 80% identical to SEQ ID NO: 2; 2) a mutant ketol-acid reductoisomerase (KARI) comprising at least one mutation or modification at positions corresponding to amino acids selected from: (a) tyrosine 35 of the Slackia exigua KARI (SEQ ID NO: 2); (b) leucine 57 of the Slackia exigua KARI (SEQ ID NO: 2); (c) arginine 58 of the S. exigua KARI (SEQ ID NO: 2); (d) glycine 60 of the Slackia exigua KARI (SEQ ID NO: 2); (e) serine 61 of the Slackia exigua KARI (SEQ ID NO: 2); (f) serine 62 of the Slackia exigua KARI (SEQ ID NO: 2); (g) serine 63 of the Slackia exigua KARI (SEQ ID NO: 2); (h) isoleucine 95 of the Slackia exigua KARI (SEQ ID NO: 2); and (i) valine 99 of the Slackia exigua KARI (SEQ ID NO: 2); 3) new isolated nucleic acid molecule encoding a mutant KARI, where the mutant KARI comprises at least one mutation or modification at positions corresponding to amino acids selected from: (a) tyrosine 35 of the Slackia exigua KARI (SEQ ID NO: 2); (b) leucine 57 of the Slackia exigua KARI; (c) glycine 60 of the Slackia exigua KARI (SEQ ID NO: 2); (d) serine 61 of the S. exigua KARI; (e) serine 63 of the Slackia exigua KARI; (f) isoleucine 95 of the Slackia exigua KARI; and (g) valine 99 of the Slackia exigua KARI; and 4) producing isobutanol, involving: (a) providing a recombinant microorganism; and (b) cultivating the recombinant microorganism in a culture medium containing a feedstock providing a carbon source, until the isobutanol is produced.