[Instability study of Escherichia coli strains containing hybrid plasmids with threonine operon fragments]. / Issledovanie nestabil'nosti shtammov Escherichia coli, soderzhashchikh gibridnye plazmidy s fragmentami treoninovogo operona.

The stability of Escherichia coli strains carrying hybrid plasmids which contain ColE1-like replicon and threonine operon genes was studied. It was shown that the main reason for instability is the loss of a plasmid. The second reason for instability is the rec-dependent recombination that leads to formation of new plasmids. All experiments where instability of strains was observed, can be quantitatively described by the model that presumes a random loss of plasmids in cell population with a frequency of about 7.10(-4), which results in diappearance of plasmid-borne cells due to their low growth rate. Instability increases during the stationary phase but it is not easy to quantitatively estimate this process.

[Interaction of RNA polymerase with hybrid plasmids carrying Escherichia coli threonine operon]. / Osobennosti vzaimodeistviia RNK-polimerazy s gibridnymi plazmidami, nesushchimi treoninovyi operon E. coli.

The RNA polymerase binding with two hybrid plasmids carrying threonine operon genes was studied. RNA polymerase binding sites were localized using nitrocellulose binding assay and electron microscopy visualization of the RNA polymerase--DNA complexes. To confirm that observed RNA polymerase binding sites are real promoters we analyzed RNA transcripts synthesized in vitro by hybridization with DNA fragments. The promoters of following genes were localized on the plasmid maps: ampicilline, threonine and RNA-primer of DNA replication. Two latter promoters bound RNA polymerase only being in the supercoiled DNA molecule. Several additional binding sites were found. The positions of some of these sites corresponded with known sites of rho-independent transcription termination.

[Cloning of threonine operon genes in Escherichia coli cells]. / Klonirovanie genov treoninovogo operona v kletkakh Escherichia coli.

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.