Evidence for the Translocation of a Chromosome Sement in Bacillus subtilis Strains Carrying the trpE26 Mutation.

The replication order of markers was studied in Bacillus subtilis strains bearing the trpE26 mutation by the use of the density transfer technique. An important difference in this order was observed in comparison with that of strain 168 T-. All markers tested of a chromosome segment extending from trpD to ilvA replicated early, after purB6 and before thr-5. Two markers flanking this region, trpE8 and citK7, replicated late as usual. The results suggested that this segment was shifted in trpE26 strains to a region closer to the origin of replication. PBS-1-mediated transduction crosses corroborated this hypothesis and revealed the position of the translocated segment. (i) Linkage was demonstrated for markers in the segment (hisH2, tryA1, met B3, ilvA2) to thr-5 and ald; (ii) aroB2 and citK7 were found to be linked; and (iii) linkage of cysB3 to thr-5 was lost in trpE26 strains. These findings made it possible to account for the characteristics of the trpE26 mutation and to propose a model explaining the fact that all trpE26+ transformants or transductants are merodiploid. The model calls for fusion of two genetic elements: two independent chromosomes, or two arms of a replicating structure. The resulting chromosome bears a long tandem duplication. Most of the features of this system of merodiploid formation can be interpreted by use of this model: the segregation pattern of the diploids, the stabilization of the unstable clones, and the length of the duplicated region. A relatively stable diploid strain was also studied by the density transfer technique. The data show that it remained diploid for the region corresponding to the translocated segment and are in agreement with the structure predicted by the model.