Reduced by-product accumulation for improved production of isobutanol

A recombinant microorganism comprising:

• a) an isobutanol producing metabolic pathway, wherein said isobutanol producing metabolic pathway comprises at least one exogenous gene or at least one overexpressed endogenous gene encoding an enzyme that catalyzes a pathway step in the conversion of pyruvate to isobutanol;
• and
• b) at least one modification which disrupts the expression or activity of an endogenous enzyme having 3-keto acid reductase activity, wherein said modification is selected from the group consisting of:
• I. a mutation of an endogenous gene encoding said endogenous enzyme having 3-keto acid reductase activity;
• and
• II. a mutation of a regulatory element associated with an endogenous gene encoding said endogenous enzyme having 3-keto acid reductase activity;
• wherein said endogenous enzyme having 3-keto acid reductase activity catalyzes the conversion of acetolactate to 2,3-dihydroxy-2-methylbutanoic acid (DH2MB).