Induction and transmission of a merodiploid condition near the terminal area of the chromosome of Bacillus subtilis.

A small fraction (about 0.5%) of the transformants for a particular marker of B. subtilis (ilvA4; most probably a deletion) were found to be relatively unstable merodiploids. They possess a redundancy of the metB-ilvA chromosome segment. When their DNA is used as donor in transformation a merodiploid condition for the whole of this segment is created in all ilvA4+ transformants. For several of the duplicated loci both copies often are of recipient strain origin. Markers originally belonging to different copies of the diploidized region can be cotransferred in PBS1-mediated transduction. The data are well in agreement with the hypothesis that the merodiploids carry a tandem duplication. An alternative hypothesis which does not call for integration of the exogenote within the recipient chromosome was also considered. Models are proposed for interpreting the segregation of the merodiploids, the transmission of the diploid state and its generation during transformation of the ilvA4 marker by wild-type DNA.