Strains of Lactococcus lactis with a partial pyrimidine requirement show sensitivity toward aspartic acid

The growth rate of the widely used laboratory strain Lactococcus lactis subsp. cremoris LM0230 was reduced if aspartic acid were present in the growth medium. The strain LM0230 is a plasmid- and phage-cured derivative of L. lactis subsp. cremoris C2, the ancestor of the original dairy isolate L. lactis subsp. cremoris NCDO712. The growth of both C2 and NCDO712 was unaffected by exogenous aspartate. Also, the growth rate of the pyrimidine auxotrophic mutants of L. lactis was affected by exogenous aspartate. The maximum observed reduction in the growth rate was similar to 35% when compared with the wild-type strain, and this varied with the concentration of aspartic acid. The observed effect of aspartate could be explained by the accumulation of the toxic pyrimidine de novo pathway intermediate, carbamoyl aspartate. Assays of the pyrimidine biosynthetic enzymes of L. lactis LM0230 showed that the partial pyrimidine requirement can be explained by a low specific activity of the pyrimidine biosynthetic enzymes. In conclusion, L. lactis LM0230 during the process of plasmid- and prophage-curing has acquired a partial pyrimidine requirement resulting in sensitivity toward aspartic acid.