SUMMARY: strains possessing the marker (splitting of the locus and a non-tandem duplication of chromosome segment Ib: ) when transformed or transduced to tryptophan independence mainly give rise to haploid cells with the genetic structure of strain 168. However, among the Trp transformants or transductants about 10% are merodiploid carrying a non-tandem duplication of segment C () while maintaining that of segment Ib. Linkage and segregation studies made it possible to determine their genetic structure, which can be represented by three different maps. In map a the copies of Ib are inverted repeats and one of them is flanked by two direct repeats of segment C; in map two Ib-C segments are inverted repeats and in map the copies of C are inverted repeats with one of them flanked by direct repeats of Ib. It is proposed that transition from map to map and then to map and vice versa, may occur by recombination between inverted repeats of either Ib or C. The merodiploids are unstable, recombination between direct repeats leading to haploid cells of 168-type structure. The models proposed for merodiploid formation call for fusion of two recipient chromosomes mediated by the donor segment and recombination between copies of a DNA sequence of the two chromosomes located in different regions. In the case of PBS-1 mediated transduction the greater length of the donor DNA segment makes it possible to obtain the merodiploids with a single recipient chromosome and this needs only a slight modification of the models. No merodiploids are found in transformation when the recipient carries a deletion of the SPβ prophage, or in transduction when both donor and recipient possess this deletion. These results indicate that the homologous sequences involved may be part of the SPβ prophage or that a sequence of bacterial DNA has a good homology with it.


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