Up-regulation of narX, encoding a putative ‘fused nitrate reductase’ in anaerobic dormant Mycobacterium bovis BCG

Mycobacterium tuberculosis and its closely related but non-pathogenic relative M. bovis Bacille Calmette-Guèrin (BCG) have the capability to adapt to anaerobiosis by shifting down from aerobic growth to a state of non-replicating persistence or dormancy. Here, we report the results of a comparative Northern analysis of 23 genes identified in the tubercle bacillus genome project that might play a role in the energy metabolism under anaerobic conditions. The expression of a majority of the genes was found to be down-regulated in the dormant BCG culture. However, the mRNA level for narX, a putative ‘fused nitrate reductase’ not found in other bacteria, was strongly up-regulated in anaerobic dormant bacilli. narX is the first transcriptionally induced gene in anaerobic dormant mycobacteria and might be a useful marker for monitoring the dormancy response in infected animals.