Abstract
An integrable plasmid, pOK4, which replicated independently in Escherichia coli was constructed for generating transcriptional fusions in vivo in Bacillus DNA. It did not replicate independently in Bacillus subtilis, but it could be made to integrate into the chromosome of B. subtilis if sequences homologous to chromosomal sequences were inserted into it. It had a selectable marker for chloramphenicol resistance and carried unique sites for EcoRI and SmaI just to the 5' side of a promoterless alpha-amylase gene from Bacillus licheniformis. When B. subtilis DNA fragments were ligated into one of these sites and the ligation mixture was used to transform an alpha-amylase-negative B. subtilis strain, chloramphenicol-resistant transformants could be isolated conveniently. Many of these were alpha-amylase positive, owing to the fusion of the plasmid amylase gene to chromosomal operons. In principle, because integration need not be mutagenic, it is possible to obtain fusions to any chromosomal operon. The site of each integration can be mapped, and the flanking sequences can be cloned into E. coli. The alpha-amylase gene can be used to detect regulated genes. We used it as an indicator to detect operons which are DNA-damage-inducible (din), and we identified insertions in both SP beta and PBSX prophages.