Instability of an arginine-overproducing mutant of Serratia marcescens and its stabilization

Arginine productivity of an arginine-producing mutant of Serratia marcescens decreased during successive batch culturing. The mutant grew more slowly than the parent strains in a minimal medium, and spontaneously produced derivatives that grew more rapidly than the mutant. A large majority of the derivatives required N-acetylglutamate or arginine for growth, due to lack of N-acetylglutamate synthase, the argA gene product. The argA1 allele carried by the mutant was found to be relatively unstable. While the mutation rate in a stable argA mutant allele was less than 1 X 10(-8) per cell per generation, that in the argA1 allele was 9 X 10(-7). The instability of the arginine productivity, therefore, was owing to both a disadvantage of the mutant in growth and a high mutability in the argA1 allele. In addition to the auxotrophs, the unstable arginine-producing mutant spontaneously produced at low frequency stable arginine-producing derivatives; among them, AT428 formed N-acetylglutamate synthase with a reduced affinity for glutamic acid. The derivative showed restored capability for propagation, and stably produced a large amount of arginine in the presence of glutamic acid or fumaric acid. By transductional analysis, the derivative was found to have acquired in the argA allele an additional mutation leading to the reduced affinity independently of the original one leading to the feedback-resistant enzyme.