Disruption of the unique B. anthracis htrA locus (encoding for the extracellular chaperone/protease HtrA) results in increased susceptibility to oxidative stress (Chitlaru et al., 2011 Molecular Microbiology 81:1542). Accordingly, we initiated a comparative transcriptomic study of an htrA mutant and its isogenic parental strain following exposure to H2O2. Data generated by high throughput RNA sequencing (Illumina Genome Analyzer), as well as direct evaluation of the pattern of expression of individual selected-genes by real-time qRT-PCR, revealed some surprising and interesting observations regarding the global effect of the htrA locus on the response to oxidative stress. A peroxide-induced increase of over 50-fold in expression of katB is observed not only in the parental strain but in the htrA mutant as well, indicating that the observed sensitivity to peroxide cannot be attributed to low levels of catalase transcription. While the levels of expression of htrA itself in the parental strain are not affected, the wild-type and the mutant share about 1200 genes similarly modulated (up or down-regulated by a factor of 2) upon peroxide exposure. Expression of 631 genes is uniquely modulated in the mutant and only 313 in the parental strain. Moreover, increasing the differential transcription factor threshold to a stringent factor of 5, demonstrates that the number of unique genes differentially expressed in the mutant strain is 190, 6 times higher than that found in the wild-type strain. These observations will be discussed in light of the functional distribution of the affected genes in the two genetic backgrounds.
