Study of chromosome rearrangements associated with the trpE26 mutation of Bacillus subtilis

Chromosome rearrangements involved in the formation of merodiploid strains in the Bacillus subtilis 168-166 system were explained by postulating the e xistence of intrachromosomal homology regions. This working hypothesis was tested by analysing sequences and restriction patterns of the, as yet uncha racterized, junctions between chromosome segments undergoing rearrangements in parent, 168 trpC2 and 166 trpE26, as well as in derived merodiploid str ains. Identification, at the Ia/Ib chromosome junction of both parent strai ns, of a 1.3 kb segment nearly identical to a segment of prophage SP beta e stablished the existence of one of the postulated homology sequences. Inspe ction of relevant junctions revealed that a set of different homology regio ns, derived from prophage SP beta, plays a key role in the formation of so- called trpE30, trpE30(+), as well as of new class I merodiploids. Analysis of junctions involved in the transfer of the trpE26 mutation, i.e. simultan eous translocation of chromosome segment C and rotation of the terminal rel ative to the origin moiety of the chromosome, did not confirm the presence of any sequence suitable for homologous recombination. We propose a model i nvolving simultaneous introduction of four donor DNA molecules, each compri sing a different relevant junction, and their pairing with the junction reg ions of the recipient chromosome. The resolution of this structure, resting on homologous recombination, would confer the donor chromosome structure t o the recipient, achieving some kind of 'transstamping'. In addition, a rat her regular pattern of inverse and direct short sequence repeats in regions flanking the breaking points could be correlated with the initial, X-ray-i nduced, rearrangement.