Analysis ofcrylAa expression insigE andsigK mutants ofBacillus thuringiensis

ThesigE andsigK genes, encoding the sporulation-specific sigma factorsσ³⁵ andσ²⁸ ofBacillus thuringiensis, were each disrupted by inserting a gene conferring resistance to kanamycin into their coding sequence. TheB. thuringiensis SigE⁻ and SigK⁻ mutant strains were blocked at different sporulation stages and were unable to sporulate. The SigE⁻ strain was blocked at stage II of sporulation, whereas the SigK⁻ strain was blocked at stage IV. The expression of acryIAa′-′lacZ transcriptional fusion was analysed in these genetic backgrounds and it was found that both sigma factors are involved in the in vivo transcription of this gene. However, the SigK⁻ strain harbouring thecryIAa gene produced amounts of toxin similar to those produced by theB. thuringiensis Spo⁺ strain. The toxins accumulated in the mother cell compartment to form a crystal inclusion which remained encapsulated within the cell wall. Thus, transcription from theσ^E-dependent promoter alone (Bt I promoter) is sufficient to support high levels of toxin production inB. thuringiensis.