Generation of deletions in pneumococcal mal genes cloned in Bacillus subtilis.

The pneumococcal recombinant plasmid pLS70, which contains two strong promoters for transcription of the malM and malX genes, is unstable when transferred to Bacillus subtilis, and it gives rise to deleted derivatives. Analysis of proteins produced by the deleted plasmids and restriction mapping of 29 different deletions showed that stabilization in B. subtilis was accompanied by deletions affecting both promoters. Plasmids containing even a single strong promoter were at a selective disadvantage. Nucleotide sequences surrounding the deletions in 10 plasmids were determined. Six different deletions occurred between directly repeated sequences of 3-13 base pairs in length, presumably by a recombination mechanism involving short homologies. Four deletions occurred between sites not contained within repeated sequences. A weak but significant similarity of an 11-base sequence was found surrounding these deletions and the corresponding points of junction in the progenitor plasmids. It is suggested that this sequence may be the recognition site for a topoisomerase-like enzyme that can produce deletions.