Cascade of extracytoplasmic function sigma factors in Mycobacterium tuberculosis:: identification of a σJ-dependent promoter upstream of sigI

A previously optimized Escherichia coli two-plasmid system was used to identify promoters recognized by RNA polymerase containing the Mycobacterium tuberculosis extracytoplasmic function (ECF) sigma factor sigma(J). The only positive clone revealed a promoter directing expression of the sigI gene encoding ECF sigma factor sigma(I). The transcriptional start point of the promoter was determined and the promoter was confirmed by an in vitro transcription assay with either E. coli or Mycobacterium smegmatis core RNA polymerase and isolated sigma(J).