[Study of RecA-independent homologous recombination and a chromosomal rearrangement in the Escherichia coli strain carrying an extended tandem duplication].

A heterozygous tandem duplication in the Escherichia coli deo operon region deoA deoB::Tn5/deoC deoD thr::Tn9 with the total length approximately 150 kb, which was obtained in the conjugational mating in the HfrH strain, was examined. By means of digestion with the NotI enzyme, pulsed-field gel electrophoresis, and the conjugational transfer of the duplication in the F- strain, the chromosomal rearrangement, which occurred in the duplication region upon its stabilization in the bacterial genome, was studied. In a more stable strain, two new NotI sites were shown to appear in the chromosomal region located close to the duplication, which might have resulted from the transposition of the IS50 sequence from Tn5. The data were also obtained indicating the possibility of secondary transposition of the chromosomal segment between the two new NotI sites (approximately 30 kb) in the region located near the duplication. With the use of rec+ and recA strains, two types of haploid and diploid segregants generated by the duplication were studied: DeoD+ (the DeoD+ allele is not expressed in the original duplication due to the polar effect of the deoB::Tn5 insertion) and DeoC DeoD. The segregation of DeoD+ clones was shown to be RecA-dependent, whereas the DeoC DeoD segregants selected on the medium that contained thymine at a low concentration (i.e., under conditions of thymine starvation) appeared at a rather high frequency. However, the relative frequency of haploid clones, which have lost the duplication, strongly decreased in the recA genome among segregants of both types.