Catalyzes the condensation of the acetyl group of acetyl-CoA with 3-methyl-2-oxobutanoate (2-oxoisovalerate) to form 3-carboxy-3-hydroxy-4-methylpentanoate (2-isopropylmalate). Carries out the first step of the leucine biosynthesis pathway. Also displays a low citramalate synthase activity, using pyruvate as substrate, but is unable to use 2-oxoglutarate. 3-methyl-2-oxobutanoate + acetyl-CoA + H2O = (2S)-2-isopropylmalate + CoA + H(+) Is not inhibited by leucine. Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 1/4. Cells lacking this gene exhibit very retarded growth in MM and the addition of isoleucine does not affect the growth of this mutant, whereas the addition of leucine appears to improve growth in MM. Belongs to the alpha-IPM synthase/homocitrate synthase family.