Single nucleotide polymorphisms that cause structural changes in the cyclic AMP receptor protein transcriptional regulator of the tuberculosis vaccine strain Mycobacterium bovis BCG alter global gene expression without attenuating growth.

Single nucleotide polymorphisms (SNPs) are present in the global transcriptional regulator cyclic AMP (cAMP) receptor protein (CRP) of the attenuated vaccine strain Mycobacterium bovis, bacillus Calmette-Guérin (BCG). We have found that these SNPs resulted in small but significant changes in the expression of a number of genes in M. tuberculosis when a deletion of the Rv3676 CRP was complemented by the BCG allele, compared to complementation by the M. tuberculosis allele. We can explain these changes in gene expression by modeling the structure of the mycobacterial protein on the known structure of CRP from Escherichia coli. Thus, the SNP change in the DNA-binding domain, Lys178, is predicted to form a hydrogen bond with the phosphate backbone of the DNA, as does the equivalent residue in E. coli, whereas Glu178 in M. tuberculosis/M. bovis does not, thus explaining the stronger binding reported for CRP of BCG to CRP-binding sites in mycobacterial DNA. In contrast, the SNP change in the nucleotide binding domain (Leu47Pro) is predicted to result in the loss of one hydrogen bond, which is accommodated by the structure, and would not therefore be expected to cause any change in function relating to cAMP binding. The BCG allele fully complemented the growth defect caused by the deletion of the Rv3676 protein in M. tuberculosis, both in vitro and in macrophage and mouse infections, suggesting that these SNPs do not play any role in the attenuation of BCG. However, they may have allowed BCG to grow better under the in vitro-selective conditions used in its derivation from the M. bovis wild type.

Point mutations in the DNA- and cNMP-binding domains of the homologue of the cAMP receptor protein (CRP) in Mycobacterium bovis BCG: implications for the inactivation of a global regulator and strain attenuation.

The genome of Mycobacterium tuberculosis H37Rv includes a homologue of the CRP/FNR (cAMP receptor protein/fumarate and nitrate reduction regulator) family of transcription regulators encoded by Rv3676. Sequencing of the orthologous gene from attenuated Mycobacterium bovis Bacille Calmette-Guérin (BCG) strains revealed point mutations that affect the putative DNA-binding and cNMP-binding domains of the encoded protein. These mutations are not present in the published sequences of the Rv3676 orthologues in M. bovis, M. tuberculosis or Mycobacterium leprae. An Escherichia coli lacZ reporter system was used to show that the M. tuberculosis Rv3676 protein binds to DNA sites for CRP, but this DNA binding was decreased or abolished with the Rv3676 protein counterparts from BCG strains. The DNA-binding ability of the M. tuberculosis Rv3676 protein was decreased by the introduction of base changes corresponding to the BCG point mutations. Conversely, the DNA binding of the BCG Rv3676 proteins from BCG strains was restored by removing the mutations. These data show that in this reporter system the point mutations present in the Rv3676 orthologue in BCG strains render its function defective (early strains) or abolished (late strains) and suggest that this protein might be naturally defective in M. bovis BCG strains. This raises the possibility that a contributing factor to the attenuation of BCG strains may be an inability of this global regulator to control the expression of genes required for in vivo survival and persistence.

Tubercle bacilli generate a novel cell wall-associated pigment after long-term anaerobic culture.

Many cases of tuberculosis result from reactivation of previously acquired latent infections. Models to study such persister forms often involve gradual depletion of oxygen during culture as poor aeration is a characteristic of non-progressive TB granulomas. Anaerobically cultured bacilli develop a thickened outer-most cell wall layer. Here, we analyzed this layer from anaerobically cultured Mycobacterium tuberculosis and Mycobacterium bovis BCG. By six weeks of anaerobiosis a pigment was detected at levels > 60-fold higher in anaerobic than aerobic bacilli. This pigment was responsible for the electron-dense appearance of the thickened cell wall layer and gave an electrospray mass spectrometry peak at 409 Da (M+Na)+ or (M+H)+. We termed this pigment APP1, anaerobically produced pigment 1, the first pigment identified in M. tuberculosis.