Fermentive production of four carbon alcohols

A recombinant microbial host cell comprising an engineered isobutanol biosynthetic pathway, wherein said isobutanol biosynthetic pathway comprises the following substrate to product conversions:

• (a) pyruvate to acetolactate;
• (b) the acetolactate from (a) to 2,3-dihydroxy-isovalerate;
• (c) the 2,3-dihydroxy-isovalerate from (b) to α-ketoisovalerate;
• (d) the α-ketoisovalerate from (c) to isobutyraldehyde;
• and
• (e) the isobutyraldehyde from (d) to isobutanol;
• wherein
• i) the substrate to product conversion of step (a) is performed by a recombinantly expressed acetolactate synthase enzyme;
• ii) the substrate to product conversion of step (b) is performed by a recombinantly expressed acetohydroxy acid isomeroreductase enzyme;
• iii) the substrate to product conversion of step (c) is performed by a recombinantly expressed acetohydroxy acid dehydratase enzyme;
• iv) the substrate to product conversion of step (d) is performed by a recombinantly expressed decarboxylase enzyme;
• and
• v) the substrate to product conversion of step (e) is performed by an alcohol dehydrogenase enzyme;
• wherein two or more enzymes of i)-v) are encoded by chimeric genes;
• and wherein said recombinant microbial host cell is capable of producing isobutanol from pyruvate through the substrate to product conversions of (a) to (e).