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J. Bacteriol., Aug 1997, 5046-5055, Vol 179, No. 16
NE Caceres, NB Harris, JF Wellehan, Z Feng, V Kapur and RG Barletta
D-Cycloserine is an effective second-line drug against Mycobacterium avium
and Mycobacterium tuberculosis. To analyze the genetic determinants of
D-cycloserine resistance in mycobacteria, a library of a resistant
Mycobacterium smegmatis mutant was constructed. A resistant clone harboring
a recombinant plasmid with a 3.1-kb insert that contained the glutamate
decarboxylase (gadA) and D-alanine racemase (alrA) genes was identified.
Subcloning experiments demonstrated that alrA was necessary and sufficient
to confer a D-cycloserine resistance phenotype. The D-alanine racemase
activities of wild-type and recombinant M. smegmatis strains were inhibited
by D-cycloserine in a concentration-dependent manner. The D-cycloserine
resistance phenotype in the recombinant clone was due to the overexpression
of the wild-type alrA gene in a multicopy vector. Analysis of a spontaneous
resistant mutant also demonstrated overproduction of wild-type AlrA enzyme.
Nucleotide sequence analysis of the overproducing mutant revealed a single
transversion (G-->T) at the alrA promoter, which resulted in elevated
beta-galactosidase reporter gene expression. Furthermore, transformants of
Mycobacterium intracellulare and Mycobacterium bovis BCG carrying the M.
smegmatis wild-type alrA gene in a multicopy vector were resistant to
D-cycloserine, suggesting that AlrA overproduction is a potential mechanism
of D-cycloserine resistance in clinical isolates of M. tuberculosis and
other pathogenic mycobacteria. In conclusion, these results show that one
of the mechanisms of D-cycloserine resistance in M. smegmatis involves the
overexpression of the alrA gene due to a promoter-up mutation.
Copyright © 1997, American Society for Microbiology
Overexpression of the D-alanine racemase gene confers resistance to D- cycloserine in Mycobacterium smegmatis
Department of Veterinary and Biomedical Sciences, University of Nebraska, Lincoln 68583-0905, USA.
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