O(6)-alkylguanine-DNA alkyltransferase DNA repair in mycobacteria: pathogenic and non-pathogenic species differ.
METHODS
DNA repair genes assist the organism in maintaining DNA integrity in the face of environmental (mutagenic) stress. The genome sequences of M. tuberculosis and M. bovis demonstrate sequences suggestive of an O(6)-alkylguanine-DNA alkyltransferase DNA repair activity similar to that seen in almost all other bacterial and eukaryotic organisms. The near ubiquitousness of this gene implies an important function.


OBJECTIVE
Our aim was to ascertain whether mycobacteria exert an alkyltransferase response to mutagen (streptozotocin) stimulation and whether alkyltransferase activity is essential for mycobacterial survival.


METHODS
Alkyltransferase activity in slow- and fast-growing mycobacterial species was determined in the presence and absence of sublethal concentrations of an alkylating agent streptozotocin. The intracellular survival and response to anti-tuberculosis drugs of an alkyltransferase knockout strain of M. bovis BCG was also determined.


RESULTS
We demonstrate the presence of O(6)-alkylguanine alkyltransferase (cellular methyltransferase activity) in mycobacterial species and that there is an inducible and constitutive form in fast-growing mycobacteria (M. smegmatis), whereas only the constitutive form exists in the pathogenic or slow-growing species (M. bovis BCG) under the conditions tested. The overall activity of the constitutive form is high. We also show that intracellular growth of M. bovis BCG in macrophages is reduced when the alkyltransferase gene is absent. The presence of alkyltransferase activity appears to assist the organism in reducing the effects of isoniazid, since interruption of the gene confers sensitivity to the drug.


CONCLUSIONS
We conclude that for the slow-growing mycobacteria, an inducible response is not essential as their ecological niche is stable and protected, but that the presence of the alkyltransferase activity confers a growth advantage in macrophages and offers some protection against antibiotics.
D001483:Base Sequence; D015139:Blotting, Southern; D004260:DNA Repair / Q000235:genetics*; D004789:Enzyme Activation; D006801:Humans; D008264:Macrophages / Q000382:microbiology; D008780:Methyltransferases / Q000235:genetics; D009163:Mycobacterium bovis / Q000201:enzymology* / Q000235:genetics; D020102:Mycobacterium smegmatis / Q000201:enzymology* / Q000235:genetics; D009169:Mycobacterium tuberculosis / Q000201:enzymology* / Q000235:genetics; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000235:genetics / Q000378:metabolism*; D013045:Species Specificity
D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase