Analysis of cryIAa expression in sigE and sigK mutants of Bacillus thuringiensis
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The sigE and sigK genes, encoding the sporulation-specific sigma factors σ35 and σ28 of Bacillus thuringiensis, were each disrupted by inserting a gene conferring resistance to kanamycin into their coding sequence. The B. 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 a cryIAa′-′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 the cryIAa gene produced amounts of toxin similar to those produced by the B. 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 the σE-dependent promoter alone (Bt I promoter) is sufficient to support high levels of toxin production in B. thuringiensis.
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