ALA biosynthesis pathway in C. glutamicum. The enzymes encoded by the corresponding genes are as follows: GdhA, glutamate dehydrogenase; GltX, glutamyl-tRNA synthetase; HemA, glutamyl-tRNA reductase; HemL, glutamate-1-semialdehyde aminotransferase (in the C5 pathway); HemA, 5-aminolevulinate synthase (in the C4 pathway). Cross in pathway, sucC and sucD, encoding succinyl-CoA synthetase, were deleted; circled gene, HemA in the C4 pathway was exogenously expressed.