A beta-galactosidase deletion mutant of Lactobacillus bulgaricus reverts to generate an active enzyme by internal DNA sequence duplication.

Several spontaneous Lac- deletion derivatives of the beta-galactosidase gene of Lactobacillus bulgaricus were analyzed for their phenotypic stability. We found that one of these mutants, lac139, carrying a deletion of 30 bp within the gene, was able to revert to a Lac+ phenotype. Genetical analysis of revertants indicated that an internal region of 72 bp was duplicated immediately next to the deletion site. The region involved in the duplication event is flanked by direct repeated sequences of 13 bp in length. Both events, the deletion and the duplication, were mediated by the presence of such short direct repeats. Enzymatic studies of the purified proteins indicated identical kinetic parameters, but showed considerable instability of the revertant protein.