The Bacillus subtilis gene senS, when present in high copy number, stimulates the expression of several extracellular protein genes during the onset of stationary phase, e.g. aprE. A novel integration vector, pINT, was constructed for transcription expression studies; it employed a unique method of promoter insert production for fusion with the lacZ reporter gene. Deletions were made of the 5' flanking region of the aprE promoter to localize the site responsible for SenS-mediated enhancement activity. pINT was used translationally fuse aprE promoter deletion fragments with the lacZ reporter gene. A site between -177 and -415 with respect to the aprE start site of transcription was found to be required for the maximal SenS-mediated transcription increase from the aprE promoter. A multicopy vector containing the senS coding region without its native negative regulation was highly unstable in B. subtilis; this was due to the expressed senS insert.