amiA is a negative regulator of acetamidase expression in Mycobacterium smegmatis

Background: The acetamidase of Mycobacterium smegmatis is a highly inducible enzyme. Expression of this enzyme is increased 100-fold when the substrate acetamide is present. The acetamidase gene is found immediately downstream of three open reading frames. Two of these are proposed to be involved in regulation. 

Results: We constructed a deletion mutant in one of the upstream ORFs (amiA). This mutant (Mad1) showed a constitutively high level of acetamidase expression. We identified four promoters in the upstream region using a β-galactosidase reporter gene. One of these (P₂) was inducible in the wild-type, but was constitutively active in Mad1. 

Conclusions: These results demonstrate that amiA encodes a negative regulatory protein which interacts with P₂. Since amiA has homology to DNA-binding proteins, it is likely that it exerts the regulatory effect by binding to the promoter to prevent transcription.