The Bacillus subtilis 168 circular chromosome yielded ten fragments on I-CeuI endonuclease digestion. I-CeuI recognizes a 26 bp sequence that is located within the gene encoding the 23S subunit of the rRNA in Chlamydomonas eugametos, Escherichia coli and Salmonella typhimurium. The precise locations of the I-CeuI sites of the B. subtilis chromosome were determined on a NotI-SfiI physical map by (i) double digestion analyses with I-CeuI and SfiI, (ii) comparison of mutant strains lacking a specific rrn operon, (iii) using an I-CeuI linking clone and (iv) analysis of nucleotide sequence data of some rrn operons. In conclusion, all the I-CeuI sites were located within the B. subtilis rrn operons and the I-CeuI sites were conserved in all the B. subtilis 168 derivatives tested. Thus, variations in size of the I-CeuI fragments must be due to genome alterations. A B. subtilis 168 strain was investigated with I-CeuI. We demonstrated that the aberrant structure was the outcome of the inversion of an ~ 1700 kb DNA segment.
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