[Cloning of threonine operon genes in Escherichia coli cells]

A set of hybrid plasmids carrying Escherichia coli threonine genes was obtained and cloned. The plasmid pBR322 was used as a vehicle. The genetic and restriction analyses showed that genes thrA and thrB were placed between SalGI and EcoRI sites on the 2.6 megadaltons DNA region. The transcription of threonine operon genes inserted in the hybrid plasmids is under the control of its own promoter. The copy number of hybrid plasmids was reverse proportional to their molecular weight and did not depend on the replicon number. Amplification of genes of threonine operon by hybrid plasmids led to 20-25-fold increase of homoserine dehydrogenase activity, encoded by thrA gene. The expression of this gene, incorporated in hybrid plasmids, was repressed by the addition of threonine and isoleucine in the culture medium.