Catalyzes the condensation of pyruvate and acetyl-coenzyme A to form (R)-citramalate. Makes part of a pathway for isoleucine biosynthesis, i.e. the citramalate-dependent pathway. Also displays a low alpha-isopropylmalate synthase activity, using 2-oxoisovalerate as substrate, but is unable to use 2-oxoglutarate. acetyl-CoA + H2O + pyruvate = (3R)-citramalate + CoA + H(+) Inhibited by isoleucine. Amino-acid biosynthesis; L-isoleucine biosynthesis; 2-oxobutanoate from pyruvate: step 1/3. Cells lacking this gene show slow growth in MM, and the addition of isoleucine or citramalate restores the growth of the mutant. Belongs to the alpha-IPM synthase/homocitrate synthase family.