An isolated mutant of a coryneform bacterium comprising a gene coding for a polypeptide having GTP-pyrophosphate kinase activity, wherein said polypeptide comprises an amino acid sequence in which one of the proteinogenic amino acids other than L-proline is present in position 38 or a corresponding or comparable position. In addition, an isolated polynucleotide encoding a polypeptide having GTP-pyrophosphate kinase enzyme activity, a vector comprising the isolated polynucleotide, a recombinant microorganism comprising the vector, and a process for preparing the recombinant coryneform bacterium is described. A method for over-expressing a GTP-pyrophosphate kinase, a method of preparing an L-amino acid, an L-lysine comprising and L-tryptophan comprising feed is also described.