Involved in the ethylmalonyl-CoA pathway for acetate assimilation. Catalyzes the reversible hydration of mesaconyl-CoA (2-methylfumaryl-CoA) to yield beta-methylmalyl-CoA ((2R,3S)-beta-methylmalyl-CoA). (2R,3S)-beta-methylmalyl-CoA = 2-methylfumaryl-CoA + H2O Optimum pH is 7.5. Strongly up-regulated during growth on acetate as sole carbon source. Cells lacking this gene are not able to grow on acetate as a sole carbon source.