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Mycobacterium microti is a member of the Mycobacterium tuberculosis complex that causes tuberculosis in voles. Most strains of M. microti are harmless for humans, and some have been successfully used as live tuberculosis vaccines. In an attempt to identify putative virulence factors of the tubercle bacilli, genes that are absent from the avirulent M. microti but present in human pathogen M. tuberculosis or Mycobacterium bovis were searched for. A minimal set of 50 bacterial artificial chromosome (BAC) clones that covers almost all of the genome of M. microti OV254 was constructed, and individual BACs were compared to the corresponding BACs from M. bovis AF2122/97 and M. tuberculosis H37Rv. Comparison of pulsed-field gel-separated DNA digests of BAC clones led to the identification of 10 regions of difference (RD) between M. microti OV254 and M. tuberculosis. A 14-kb chromosomal region (RD1(mic)) that partly overlaps the RD1 deletion in the BCG vaccine strain was missing from the genomes of all nine tested M. microti strains. This region covers 13 genes, Rv3864 to Rv3876, in M. tuberculosis, including those encoding the potent ESAT-6 and CFP-10 antigens. In contrast, RD5(mic), a region that contains three phospholipase C genes (plcA to -C), was missing from only the vole isolates and was present in M. microti strains isolated from humans. Apart from RD1(mic) and RD5(mic) other M. microti-specific deleted regions have been identified (MiD1 to MiD3). Deletion of MiD1 has removed parts of the direct repeat region in M. microti and thus contributes to the characteristic spoligotype of M. microti strains.

The genus mycobacteria includes two important human pathogens Mycobacterium tuberculosis and Mycobacterium lepra. The former is reputed to have the highest annual global mortality of all pathogens. Their slow growth, virulence for humans and particular physiology makes these organisms extremely difficult to work with. However the rapid development of mycobacterial genomics following the completion of the Mycobacterium tuberculosis genome sequence provides the basis for a powerful new approach for the understanding of these organisms. Five further genome sequencing projects of closely related mycobacterial species with differing host range, virulence for humans and physiology are underway. A comparative genomic analysis of these species has the potential to define the genetic basis of these phenotypes which will be invaluable for the development of urgently needed new vaccines and drugs. This minireview summarises the different techniques that have been employed to compare these genomes and gives an overview of the wealth of data that has already been generated by mycobacterial comparative genomics.

The nucleotide sequence of cosmid B1790, carrying the Rif-Str regions of the Mycobacterium leprae chromosome, has been determined. Twelve open reading frames were identified in the 36716bp sequence, representing 40% of the coding capacity. Five ribosomal proteins, two elongation factors and the β and β' subunits of RNA polymerase have been characterized and two novel genes were found. One of these encodes a member of the so-called ABC family of ATP-binding proteins while the other appears to encode an enzyme involved in repairing genomic lesions caused by free radicals. This finding may well be significant as M. leprae, an intracellular pathogen, lives within macrophages. [ABSTRACT FROM AUTHOR]

In an attempt to unify the genetic and biological research on Mycobacterium leprae, the aetiological agent of leprosy, a cosmid library was constructed and then ordered by a combination of fingerprinting and hybridization techniques. The genome of M. leprae is represented by four contigs of overlapping clones which, together, account for nearly 2.8Mb of DNA. Several arguments suggest that the gaps between the contigs are small in size and that virtually complete coverage of the chromosome has been obtained. All of the cloned M. leprae genes have been positioned on the contig maps together with the 29 copies of the dispersed repetitive element, RLEP. These have been classified into four groups on the basis of differences in their organization. Several key housekeeping genes were identified and mapped by hybridization with heterologous probes, and the current genome map of this uncultivable pathogen comprises 72 loci. [ABSTRACT FROM AUTHOR]