Fermentive production of four carbon alcohols

A method for the production of isobutanol comprising:

• 1) providing a recombinant yeast host cell comprising genes encoding an engineered isobutanol biosynthetic pathway, wherein said isobutanol biosynthetic pathway increases the production of isobutanol as compared to naturally occurring amounts of isobutanol made by unmodified yeast and comprises the following substrate to product conversions:
• (a) 2,3-dihydroxyisovalerate to α-ketoisovalerate,
• (b) the α-ketoisovalerate from (a) to isobutyraldehyde, and
• (c) the isobutyraldehyde from (b) to isobutanol;
• wherein
• (i) the substrate to product conversion of step (a) is performed by a recombinantly expressed acetohydroxy acid dehydratase enzyme;
• (ii) the substrate to product conversion of step (b) is performed by a recombinantly expressed decarboxylase enzyme;
• and
• (iii) the substrate to product conversion of step (c) is performed by a recombinantly expressed alcohol dehydrogenase enzyme;
• 2) contacting the recombinant yeast host cell of 1) with a fermentable carbon substrate in a fermentation medium under conditions whereby isobutanol is produced through the substrate to product conversions of (a) to (c);
• and
• 3)recovering isobutanol produced in 2),
• wherein the recombinantly expressed enzymes of (i) to (iii) are expressed in the cytosol, and said recombinant yeast host cell is capable of producing isobutanol through the substrate to product conversions of (a) to (c).