| Abstract: | Mutants, resistant to threonine analogue, DL-alpha-amino-beta-hydroxyvaleric acid, were obtained after the treatment of Escherichia coli K-12 RelA- cells with nitrosoguanidine, and among them the strain with maximal threonine production (about 3g/l) was selected. Genetic and biochemical analysis of the producer has revealed the dependency of the threonine production on at least three mutations. The mutation in the thrA gene disturbs retroinhibition of homoserine dehydrogenase by threonine. The mutation in the ilvA gene decreases the activity of threonine deaminase, and thus results in partial isoleucine auxotrophy, and finally, the reversion in the relA gene restores the stringent amino acid control of RNA synthesis in threonine producer cells. The role of relA gene in threonine production was demonstrated by comparing pairs of strains differing from one another in the allelic state of the relA gene. The level of threonine synthesis (its intra- and extracellular concentrations) during moderate isoleucine starvation in RelA+ cells 2-3 times as high as in RelA- cells. The presence of relA+ allele is found to result in the increase of the cell resistance to DL-alpha-amino-beta-hydroxyvaleric acid. |
