GENETIC-STUDY OF THE MECHANISM OF TANDEM DUPLICATION FORMATION IN CONJUGATIONAL CROSSES IN ESCHERICHIA-COLI K-12

The formation of heterozygous tandem duplications of the deo operon wa s studied in conjugational matings HfrH deoA deoB::Tn5 thyA x HfrH deo C deoD thyA. When the HfrH deoC deoD thr::Tn9 car::Tn10 thyA donor str ain was used, the thr::Tn9 and car::Tn10 transposon insertions linked to the deo operon were integrated into some duplications. Duplications carrying the recipient thr(+) and car(+) alleles in the duplicated st ate were not found among duplications that did not include the thr::Tn 9 and car::Tn10 donor markers. These data indicate that heterozygous d eo operon duplications are primarily formed directly during the recomb inational interaction between chromosomes in the merozygote, but not o n the basis of preexisting sister duplications in the recipient strain . Thus, genetic recombination occurring during the process of conjugat ion in E. coli induces both symmetrical and unequal crossing over. Nev ertheless, when the HfrH deoA deoB::Tn5 thr::Tn9 car::Tn10 thyA strain was used as a recipient, a deo operon duplication containing thr::Tn9 and car::Tn10 markers was found in the homozygous state. Consequently , some heterozygous duplications can also be formed on the basis of pr eexisting sister duplications.