Studies on the size of the diploid region in Bacillus subtilis merozygotes from strains carrying the trpE26 mutation.

Simultaneous selection of transformants for trpE26 and a second unliked marker of B. subtilis in many cases yields double heterogenotic clones. Several chromosome areas analyzed in this way were found to be involved in the diploid condition. Diploids for areas on the left hand side of trpE26 on the map (and as near as the aroB locus) are in general unstable while stable merodiploids can be obtained for areas on the right hand side of this marker (as far as the ilvA locus). Merozygotes for regions other than the "aromatic segment" are also formed by transformation of already diploid (stable and unstable) clones. Stable diploids give rise to new heterogenotes only for markers on the right hand side of trpE26. Through reversion of untransformed markers in unstable and stable diploids it was found that these clones are homodiploid for loci situated at a long distance from (or between) the areas which were involved in the transformation. This indicates that the diploid state covers a continuous segment of the chromosome, the length of which can be determined. The segregation pattern of unstable multiple merodiploids suggests that exchange of genetic material must take place between the two homologous regions. The data presented are in agreement with the hypothesis that the merodiploids possess a very long duplication on their chromosome. In the case of the stable clones this duplication is shorter.