NOVELTY - Converting acetolactate to dihydroxy-isovalerate, comprising: (a) providing a microbial host cell comprising genetic construct encoding a polypeptide having ketol-acid reductoisomerase specific activity greater than that of the specific activity of an Escherichia coli ketol-acid reductoisomerase; and (b) contacting the host cell of (a) with acetolactate where 2,3-dihydroxy-isovalerate is produced.    USE - The methods and host cell are useful for converting acetolactate to dihydroxy-isovalerate; for producing isobutanol; and identifying and isolating of the genetic construct encoding the ketol-acid reductoisomerase enzyme (all claimed).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are: (1) a method for the production of isobutanol, comprising: (a) providing a recombinant microbial host cell comprising the following genetic constructs: (1) at least one genetic construct encoding an acetolactate synthase enzyme of the conversion of pyruvate to acetolactate (pathway step a); (2) at least one genetic construct encoding a ketol-acid reductoisomerase enzyme specific activity of greater than 1.1 mu mol/minute/mg based on purified protein as measured by the NADPH consumption assay, run under the following conditions: (i) pH of about 7.5; (ii) a temperature of 22.5 degrees C; and (iii) greater than 10 mM potassium for the conversion of (S)-acetolactate to 2,3-dihydroxyisovalcrate, (pathway step b); (3) at least one genetic construct encoding an acetohydroxy acid dehydratase for the conversion of 2,3-dihydroxyisovalerate to alpha -ketoisovalerate, (pathway step c); (4) at least one genetic construct encoding a branched-chain keto acid decarboxylase, of the conversion of alpha -ketoisovalerate to isobutyraldehyde, (pathway step d) at least one genetic construct encoding a branched- chain alcohol dehydrogenase for the conversion of isobutyraldehyde to isobutanol (pathway step e); and (b) growing the host cell of (a) under conditions where iso-butanol is produced; (2) a recombinant host cell comprising the ketol-acid reductoisomerase enzyme having the specific activity greater than the specific activity of the Escherichia coli ketol-acid reductoisomerase; and (3) a method for identifying and isolating of the genetic construct encoding the ketol-acid reductoisomerase enzyme having the specific activity of greater than 1.1 mu mol/minute/mg based on purified protein as measured by the NADPH consumption assay, run under the following conditions: (i) pH of about 7.5; (ii) the temperature of about 22.5 degrees C; and (iii) greater than 10 mM potassium; comprising: (a) identifying bacterial species having a doubling time shorter than that of E. coli when grown in M9 minimal medium; (b) screening the bacterial species of (a) for ketol-acid reductoisomerase activity to identify active bacterial species; (c) probing the genomic DNA of the active bacterial species of (b) with nucleic acid sequences having homology to genetic constructs known to encode the ketol-acid reductoisomerase to identify and isolate genetic constructs encoding the ketol-acid reductoisomerase from the active bacterial species; (d) amplifying and expressing the genetic constructs encoding the ketol-acid reductoisomerase from the active bacterial species; and (e) screening the expressed genetic constructs of (d) for those having the specific activity of greater than 1.1 mu mol/minute/mg based on purified protein as measured by the NADPH.