CHARACTERIZATION OF THE CHROMOSOMES OD BACILLUS-SUBTILIS MERODIPLOID STRAINS BY QUANTITATIVE DNA-DNA HYBRIDIZATION

The position of junctions and the extent of the duplicated chromosomal regions in Bacillus subtilis merodiploid strains were studied by quan titative DNA-DNA hybridization. We describe a method which allows (i) the identification of genes present in two copies per chromosome and ( ii) the measurement of the amount of additional DNA in chromosomes wit h relatively large duplicated regions (about 10% or more). Analysis of previously described B. subtilis merodiploid strains GSY1127, GSY1800 and GSY1835 revealed that the duplicated segments represent 29+/-2%, 7+/-2% and 13+/-2% of the chromosome, respectively. Small discrepancie s between these and previous genetic linkage data are discussed. Suppo rt for a role of prophage SP beta in the formation of merodiploid GSY1 835 is provided. In conclusion, the described method confirmed the gen etic maps of the merodiploids previously obtained by transduction and transformation crosses and showed that a duplication of a segment is n ot accompanied by large deletions of other chromosomal regions, provid ing direct evidence that a cell can accommodate genomes of substantial ly increased site.