Salmonella strains harboring tandem chromosomal duplications have been identified following selection for expression of a histidine biosynthetic gene whose promoter is deleted. In such strains, tandem duplications fuse the selected his gene to "foreign" regulatory elements, thereby allowing gene expression. Selection is made for hisD+ activity in deletion strain hisOG203. Among the revertants, strains harboring tandem chromosomal duplications have been identified by a number of their properties. (1) Their HisD+ phenotype is genetically unstable. (2) Such instability is dependent on recombination (recA) activity. (3) Genetic tests demonstrate that these strains are merodiploid for large regions (up to 25%) of the Salmonella genome. (4) Recipient strains that inherit the HisD+ phenotype of these duplication-carrying revertants also inherit the donor's merodiploid state. (5) In certain revertants the functional hisD+ gene and the sequence which promotes merodiploid transductant formation are linked to chromosomal markers located far from the normal his region. Previous reports have concluded that the instability of strains isolated by this selection is due to translocation of the hisD+ gene to an extrachromosomal element (the pi-histidine factor). We believe that in all strains we have tested (33 independent isolates) instability can better be accounted for as due to tandem duplication events which permit expression of hisD. At least two mechanisms are responsible for duplication formation. One mechanism is dependent on recombination function and generates identical revertants having a duplication of 16% of the chromosome. A second mechanism operates independently of recombination activity; individual duplications produced by this process have variable endpoints.
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