Two sigma factors, sigma 35 and sigma 28, direct transcription from the Bt I and Bt II promoters of the cryIA(a) gene of Bacillus thuringiensis; this gene encodes a lepidopteran-specific crystal protoxin. These sigma factors were biochemically characterized in previous work (K. L. Brown and H. R. Whiteley, Proc. Natl. Acad. Sci. USA 85:4166-4170, 1988; K. L. Brown and H. R. Whiteley, J. Bacteri...