Isolation and characterization of chromosomal promoters of Streptococcus salivarius subsp. thermophilus

A promoter probe vector, pTG244, was constructed with the aim of isolating transcription initiation signals from Streptococcus thermophilus (Streptococcus salivarius subsp. thermophilus). pTG244 is based on the Escherichia coli-streptococcus shuttle vector pTG222, into which the promoterless chloramphenicol acetyltransferase gene of Bacillus pumilus (cat-86) was cloned. Random Sau3A fragments from the S. thermophilus A054 chromosomal DNA were cloned upstream of the cat-86 gene by using E. coli as the host. The pool of recombinant plasmids were introduced into S. thermophilus and Lactococcus lactis subsp. lactis in order to search for promoter activity in these hosts. For S. thermophilus, it was necessary to first select erythromycin-resistant transformants and then to screen for chloramphenicol resistance among these. Direct selection of chloramphenicol-resistant clones was, however, possible in L. lactis subsp. lactis. Six fragments exhibiting promoter activity were characterized in S. thermophilus by measuring the levels of cat-86 transcription and/or chloramphenicol acetyltransferase specific activity. Three of the promoter-carrying fragments were sequenced. The 5' ends of their corresponding mRNAs were determined by S1 mapping and shown to correspond to a purine residue in all cases. Upstream from these potential transcription start points, sequences homologous to the E. coli sigma 70 and the Bacillus subtilis vegetative sigma 43 (or sigma A) consensus promoters were identified.