No abstract

The inducible acetamidase of Mycobacterium smegmatis NCTC 81 59 is expressed at high levels in the presence of a suitable inducer, such as acetamide. The gene and 1.5 kb of upstream sequence had previously been sequenced. A further 14 kb of upstream sequence has now been determined, containing an additional ORF on the opposite strand to the acetamidase gene. This ORF has significant homologies to genes encoding regulatory proteins involved in amidase expression in other organisms. Restriction fragments from the 4 kb region were subcloned into a promoter-probe shuttle vector to locate the approximate region of the acetamidase promoter and investigate the mechanism of regulation. An inducible promoter was found to lie in the 1.4 kb region situated 1-5 kb upstream from the acetamidase coding region. Expression of the acetamidase was studied at the protein and mRNA levels. Using immunoblotting, induction of the enzyme was demonstrated in minimal medium containing succinate plus acetamide, but not in a richer medium (Lemco broth) plus acetamide, confirming that regulation of acetamidase expression is mediated by both positive and negative control elements. Afier induction by acetamide, an increase above basal level could be detected after 1 h for both protein levels (using ELISA) and mRNA levels (using Northern blot analysis), indicating that control of expression is at the mRNA level. The size of the mRNA transcript detected was approximately 1.2 kb, the size of the acetamidase coding region. Since no promoter was identified immediately upstream of the coding region, this raises the possibility that a larger, primary transcript (possibly polycistronic) is cleaved to produce a stable form encoding the acetamidase protein.1

SummaryThe functions of OmpATb, the product of the ompATb gene of Mycobacterium tuberculosis and a putative porin, were investigated by studying a mutant with a targeted deletion of the gene, and by observing expression of the gene in wild-type M. tuberculosis H37Rv by real-time polymerase chain reaction (PCR) and immunoblotting. The loss of ompATb had no effect on growth under normal conditions, but caused a major reduction in ability to grow at reduced pH. The gene was substantially upregulated in wild-type bacteria exposed to these conditions. The mutant was impaired in its ability to grow in macrophages and in normal mice, although it was as virulent as the wild type in mice that lack T cells. Deletion of the ompATb gene reduced permeability to several small watersoluble substances. This was particularly evident at pH 5.5; at this pH, uptake of serine was minimal, suggesting that, at this pH, OmpATb might be the only functioning porin. These data indicate that OmpATb has two functions: as a pore-forming protein with properties of a porin, and in enabling M. tuberculosis to respond to reduced environmental pH. It is not known whether this second function is related to the porin-like activity at low pH or involves a completely separate role for OmpATB. The involvement with pH is likely to contribute to the ability of M. tuberculosis to overcome host defence mechanisms and grow in a mammalian host.

The acetamidase of Mycobacterium smegrnatis NCTC 8159 was purified, and the sequences of its amino-terminus and of two peptides obtained by proteolysis of the protein were obtained. A DNA fragment including the amidase structural gene was cloned in Escherichia coli, using oligonucleotide probes designed on the basis of the peptide sequences and a codon usage table calculated from published sequences of nine protein-antigen-encoding genes of the Mycobacterium tuberculosis complex. Sequence analysis of the cloned DNA revealed that the amidase gene encoded 406 amino acid residues. The nucleotide sequence close to and upstream of the amidase gene contained a probable ribosome-binding site but no identifiable promoter sequences. Three additional potential open-reading frames were found upstream of and very close to the amidase gene, with consensus ' -35' and ' -10' promoter sites between the first and second of these. It is hoped that the highly inducible expression of the acetamidase gene can be exploited to allow regulated expression of other genes cloned in mycobacteria.

Introduction 3. The mycobacterial envelope 3.1. The arrangement of mycolates 3.2. Evidence for the outer lipid permeability barrier 3.3. The permeability of the outer lipid barrier 4. Porins in mycobacteria 4.1. Porin-like protein of Mycobacterium tuberculosis 4.2. Porins in other slow-growing mycobacteria 4.3. Access of molecules to mycobacteria 5. A mycobacterial capsule 5.1. Permeability and the capsule 6. Perspective 7. Acknowledgement 8. References