The high sequence identity among the Mycobacterium bovis and Mycobacterium tuberculosis genomes contrasts with the physiological differences reported between these pathogens, suggesting that variations in gene expression may be involved. In this study, microarray hybridization was used to compare the total transcriptome of M. bovis and M. tuberculosis, during the exponential phase of growth. Differential expression was detected in 258 genes, representing a 6% of the total genome. Variable genes were grouped according to functional categories. The main variations were found in genes encoding proteins involved in intermediary metabolism and respiration, cell wall processes, and hypothetical proteins. It is noteworthy that, compared to M. tuberculosis, the expression of a higher number of transcriptional regulators were detected in M. bovis. Likewise, in M. tuberculosis we found a higher expression of the PE/PPE genes, some of which code for cell wall related proteins. Also, in both pathogens we detected the expression of a number of genes not annotated in the M. tuberculosis H37Rv or M. bovis 2122 genomes, but annotated in the M. tuberculosis CDC1551 genome.

Our results provide new evidence concerning differences in gene expression between both pathogens, and confirm previous hypotheses inferred from genome comparisons and proteome analysis. This study may shed some new light on our understanding of the mechanisms relating to differences in gene expression and pathogenicity in mycobacteria.