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A fragment of diphtheria toxin (tox) gene from beta 45 phage DNA was cloned on pUC19 plasmid in E. coli cells. The fragment is coding for toxA fragment of the toxin and contains the control region of the tox gene. The tox gene promoter is active in E. coli. The toxA protein is found mainly in periplasm of E. coli cells. The protein is enzymatically active in ADP-ribosilation of elongation factor 2 from eucaryotic cells. An in frame toxA-lacZ' fusion was constructed on pUC8 plasmid. The hybrid protein expresses both toxA and lacZ' activities. Two or seven base pairs were deleted from the central part of toxA gene by means of S1 nuclease digestion. Translation of hybrid toxA-lacZ' mRNA should be terminated downward the delections due to the frameshifts caused by them. Nevertheless, a functionally active alpha-peptide of beta-galactosidase is expressed by both the deletion fusions. The existence of another translational start site functioning in E. coli and located inside 3'-end region of toxA mRNA is suggested.

The transforming activity of DNA from Bacillus subtilis 168, Bac. subtilis W23, Bac. subtilis NRS and Bac. aterrimus after the EcoRI restrictase treatment was studied. The auxotrophic strains of Bac. subtilis 168 were used as recipients in bacterial transformation. The transforming activity for different markers decreased up to 0.001--6 per cent from the initial level for different Bacilli DNAs. In some cases the sensitivity of the same marker from different Bacilli differed in more than 50 times. Bac. subtilis and Bac. aterrimus have demonstrated the maximal differences. Such differences can be used for the identification of close related Bacilli.

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.

Escherichia coli DNA with molecular weight of 20 - 10(6) daltons was digested by restriction endonuclease EcoR1, and the transforming activity of resctricts was studied. The transforming activity of restricts for two markers (Leu and Arg) was 2-5 fold increased, for two other markers (Thr and His) was not changed, and for one marker (Pro) was completely absent. The molecular weight of E. coli DNA restricts was 7,5-10(6) daltons. An increase and a decrease of the transforming activity for different markers appeared to be the result of two effects: 1) more efficient uptake of low molecular weight DNA into the cell and 2) the inactivation of markers as a result of location close to EcoR1 induced break.

We studied Bacillus thuringiensis var galleriae, strain 612 plasmids. B. thuringiensis cells contain double-stranded plasmid DNA molecules (ranging of about 12% from total DNA content) with buoyant density 1.59 g/cm3. Plasmid DNA content was constant during the exponential and stationary phases of bacterial growth. The plasmid fractions consist of DNA molecules with molecular weights of 5.9 x 10(6), 10.0 x 10(6), and 110.9 x 10(6) daltons (pVD1, pVD2 pVD3, respectively). Endonuclease EcoRI cuts the plasmids pVD2 and pVD3 into two and four fragments, respectivelyy, but pVDI seemed to be resistent to EcoRI treatment. We found that pVD2 and pVD3 plasmids contain a common DNA fragment with the molecular weight of 6.7 x 10(6) dalton as it was shown by restriction analysis. In contrast, the same plasmids contain the common fragment with molecular weight of 7.5 x 10(6) dalton as shown by heteroduplex analysis. Plasmid pVD3 has a transposon-like structure.

Centrifugation of Bacillus thuringiensis 351 DNA in CsCl-ethidium bromide density gradient has revealed, besides the main band of chromosomal DNA (p = 1.56 g/cm2), some additional bands with higher density, which usually correspond cicle superhelix DNAs, are revealed. 6 discrete bands are observed under electrophoresis of total fraction of minor DNA bands, which suggests, that the preparation contains cicle DNAs of different size. The treatment with DNAse results in the appearance of 3 bands corresponding to opened cicle DNA forms, their molecular weight being 12-10(6)--4-10(6) daltons. Cicle super-helix and opened cicle DNA forms are found in minor fraction by means of electrone microscopy. Statistical analysis has revealed the presence of at least three types of cicle molecules of different size. The variant 351-10, free of extrachromosomal elements, is isolated after ethidium bromide treatment at high temperature. Possible mechanisms of cicle DNAs formation are discussed. Biological function of DNA extrachromosomes fragments in Bacillus thuringiensis is still obscure.