The immune inhibitor A (InhA) metalloprotease from Bacillus thuringiensis specifically cleaves antibacterial proteins produced by the insect host, suggesting that it may contribute to the overall virulence of B. thuringiensis. The transcriptional regulation of the inhA gene in both B. thuringiensis and Bacillus subtilis was investigated. Using a transcriptional inhA'-lacZ fusion, it was shown that inhA expression is activated at the onset of sporulation. However, the transcriptional start site of inhA is similar to sigma (A)-dependent promoters, and deletion of the sporulation-specific sigma factors sigma (F) or sigma (E) had no effect on inhA expression in B. subtilis. The DNA region upstream from inhA contains two genes encoding polypeptides similar to the SinI and SinR regulators of B. subtilis. SinR is a DNA-binding protein regulating gene expression and SinI inhibits SinR activity. Overexpression of the sin genes affects the expression of the inhA'-lacZ transcriptional fusion in B. thuringiensis: early induction of inhA expression was observed when sinI was overexpressed, whereas inhA expression was reduced in a strain overexpressing sinR, suggesting that inhA transcription is repressed, directly or indirectly, by SinR, inhA transcription was greatly reduced in B. thuringiensis and B. subtilis spo0A mutants. Analysis of the inhA'-lacZ expression in abrB and akrB-spo0A mutants of B. subtilis indicates that the Spo0A-dependent regulation of inhA expression depends on AbrB, which is known to regulate expression of transition state and sporulation genes in B. subtilis.
