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Open Access Research article

Transcriptome analysis identifies Bacillus anthracis genes that respond to CO2 through an AtxA-dependent mechanism

Andrew T McKenzie1, Andrei P Pomerantsev13*, Inka Sastalla1, Craig Martens2, Stacy M Ricklefs2, Kimmo Virtaneva2, Sarah Anzick2, Stephen F Porcella2 and Stephen H Leppla1

Author Affiliations

1 Microbial Pathogenesis Section, Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, MD 20892, USA

2 Genomics Unit, Research Technologies Section, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, Hamilton, MT 59840, USA

3 National Institutes of Health, 33 North Drive, Building 33, Room 1W20B, Bethesda, MD 20892, USA

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BMC Genomics 2014, 15:229  doi:10.1186/1471-2164-15-229

Published: 25 March 2014

Abstract

Background

Upon infection of a mammalian host, Bacillus anthracis responds to host cues, and particularly to elevated temperature (37°C) and bicarbonate/CO2 concentrations, with increased expression of virulence factors that include the anthrax toxins and extracellular capsular layer. This response requires the presence of the pXO1 virulence plasmid-encoded pleiotropic regulator AtxA. To better understand the genetic basis of this response, we utilized a controlled in vitro system and Next Generation sequencing to determine and compare RNA expression profiles of the parental strain and an isogenic AtxA-deficient strain in a 2 × 2 factorial design with growth environments containing or lacking carbon dioxide.

Results

We found 15 pXO1-encoded genes and 3 chromosomal genes that were strongly regulated by the separate or synergistic actions of AtxA and carbon dioxide. The majority of the regulated genes responded to both AtxA and carbon dioxide rather than to just one of these factors. Interestingly, we identified two previously unrecognized small RNAs that are highly expressed under physiological carbon dioxide concentrations in an AtxA-dependent manner. Expression levels of the two small RNAs were found to be higher than that of any other gene differentially expressed in response to these conditions. Secondary structure and small RNA-mRNA binding predictions for the two small RNAs suggest that they may perform important functions in regulating B. anthracis virulence.

Conclusions

A majority of genes on the virulence plasmid pXO1 that are regulated by the presence of either CO2 or AtxA separately are also regulated synergistically in the presence of both. These results also elucidate novel pXO1-encoded small RNAs that are associated with virulence conditions.

Keywords:
Bacillus anthracis; pXO1; Illumina; RNA sequencing; AtxA; Toxin production; Small RNA; Gene-environment interaction