S-layer variation in Bacillus stearothermophilus PV72 is based on DNA rearrangements between the chromosome and the naturally occurring megaplasmids

Bacillus stearothermophilus PV72 expresses different S-layer genes (sbsA an d sbsB) under different growth conditions. No stretches of significant sequ ence identity between sbsA and sbsB were detected. In order to investigate S-layer gene regulation in B. stearothermophilus PV72, we characterized the upstream regulatory region of sbsA and sbsB by sequencing and primer exten sion analysis. Both genes are transcribed from unique but different promote rs, independently of the growth phase. Localization of sbsB in the sbsA-exp ressing strain PV72/p6 revealed that the coding region of the second S-laye r gene sbsB is located not on the chromosome but on a natural megaplasmid o f the strain, whereas the upstream regulatory region of sbsB was exclusivel y detected on the chromosome of PV72/p6. For sbsB expression, the coding re gion has to be integrated into the chromosomally located expression site. A fter the switch to sbsB expression, the sbsA coding region was removed from the chromosome but could still be detected on the plasmid of the sbsB-expr essing strain PV72/p2. The sbsA upstream regulatory region, however, remain ed on the chromosome. This is the first report of S-layer variation not cau sed by intrachromosomal DNA rearrangements, but where variant formation dep ends on recombinational events between the plasmid and the chromosome.