Involved in the ethylmalonyl-CoA pathway, a new acetyl-CoA assimilation strategy that operates in a number of bacteria and replaces the glyoxylate cycle. Catalyzes the oxidation of (2S)-methylsuccinyl-CoA to yield mesaconyl-(C1)-CoA. Highly specific for (S)-methylsuccinyl-CoA. (2S)-methylsuccinyl-CoA + H(+) + oxidized [electron-transfer flavoprotein] = 2-methylfumaryl-CoA + reduced [electron-transfer flavoprotein] Binds 1 FAD per subunit. Optimum pH is 9.2. Homodimer. Cells lacking this gene are unable to utilize acetate or acetoacetate as sole carbon source, but grow on succinate or propionate plus bicarbonate. Belongs to the acyl-CoA dehydrogenase family.