Background: The mce4 operon is one of the four homologues of mammalian cell entry (mce) operons of Mycobacterium tuberculosis. The mce4A (Rv3499c) gene within this operon is homologous to mce1A (Rv0169), that has a role in host cell invasion by M. tuberculosis. Our earlier reports show that mce4 operon is expressed during the stationary phase of growth of the bacillus in culture and during the course of infection in mammalian hosts. M. tuberculosis carrying mutation in mce4 operon shows growth defect and reduced survival in infected mice. However, the intracellular localization of Mce4A protein and its direct role in cell entry or survival of the bacillus has not been demonstrated so far.

BackgroundThe mce operons play an important role in the entry of M. tuberculosis into macrophages and non-phagocytic cells. Their non-redundant function as well as complex regulation is implied by the phenotype of mce mutants. Recently, mce1 operon was found to extend over 13 genes, fadD5 (Rv0166) being the first gene of the operon. The presence of a non-coding sequence of 200 base pairs between Rv0166 and Rv0167 is peculiar to mce1 among the four mce operons of M.tuberculosis. We have examined the function of this region.ResultsWe predicted putative promoter activity of the 200 base pairs of non-coding, intergenic region between Rv0166 and Rv0167 in silico using MEME software and designate it as intergenic promoter, IGPr. We demonstrate both promoter activity and a putative negative regulatory function of this fragment by reporter assays carried out in the surrogate host M.smegmatis. We find that the repressive elements not only control the native promoter but also repress a heterologous promoter of M.smegmatis. The higher activity of the intergenic promoter in a clinical isolate in comparison with the wild type sequence from M.tuberculosis H37Rv could be correlated with a point mutation within the negative element. We have mapped two transcription start sites for mce1 operon both of which are utilized in M.tuberculosis H37Rv as well as the clinical isolate VPCI591. Our studies show that the promoter activity in the non-coding region is relevant not only in reporter gene expression but also in the expression of mce1 operon in M. tuberculosis cells grown in synthetic medium.ConclusionThe mce operon of M.tuberculosis H37Rv potentially can be transcribed from two promoters P1 and P2, former mapping upstream of Rv0166 and the latter in the non-coding intergenic region between Rv0166 and Rv0167. The transcription initiation from P1 results in a transcript with Rv0166 while that from P2 will be without it. The sequences between the translation start site of Rv0167 and the promoter P2 have a negative regulatory role, as point mutation within the sequence leads to enhanced activity of P2 as well as a heterologous promoter from M.smegmatis. The mutation detected in the clinical isolate VPCI591 therefore behaves like a gain-of-function mutation.

BackgroundThe presence of four mammalian cell entry (mce) operons in Mycobacterium tuberculosis suggests the essentiality of the functions of the genes in these operons. The differential expression of the four mce operons in different phases of in vitro growth and in infected animals reported earlier from our laboratory further justifies the apparent redundancy for these genes in the genome.Here we investigate the extent of polymorphism in eight genes in the mce1 and mce4 operons of M. tuberculosis from four standard reference strains (H37Rv, H37Ra, LVS (Low Virulent Strain) and BCG) and 112 clinical isolates varying in their drug susceptibility profile, analysed by direct sequencing and Sequenom MassARRAY platform.ResultsWe discovered 20 single nucleotide polymorphisms (SNPs) in the two operons. The comparative analysis of the genes of mce1 and mce4 operons revealed that yrbE1A [Rv0167] was most polymorphic in mce1 operon while yrbE4A [Rv3501c] and lprN [Rv3495c] had the highest number of SNPs in the mce4 operon. Of 20 SNPs, 12 were found to be nonsynonymous and were further analysed for their pathological relevance to M. tuberculosis using web servers PolyPhen and PMut, which predicted five deleterious nonsynonymous SNPs. A mutation from proline to serine at position 359 of the native Mce1A protein was most deleterious as predicted by both PolyPhen and PMut servers. Energy minimization of the structure of native Mce1A protein and mutated protein was performed using InsightII. The mutated Mce1A protein showed structural changes that could account for the effects of this mutation.ConclusionsOur results show that SNPs in the coding sequences of mce1 and mce4 operons in clinical isolates can be significantly high. Moreover, mce4 operon is significantly more polymorphic than mce1 operon (p < 0.001). However, the frequency of nonsynonymous substitutions is higher in mce1 operon and synonymous substitutions are more in mce4 operon. In silico modeling predict that nonsynonymous SNP at mce1A [Rv0169], a virulence gene could play a pivotal role in causing functional changes in M. tuberculosis that may reflect upon the biology of the bacteria.

Lipoproteins are known to be effective immunogens and affect both innate and adaptive immunity. The lprN gene of Mycobacterium tuberculosis has been predicted to encode for a putative lipoprotein in silico. Here, we studied its function as an immunogen by in vivo studies in mice. The recombinant LprN protein, expressed and purified in Escherichia coli, triggered a cell-mediated immune response in BALB/c mice. This was observed by significantly higher T-cell proliferation and increased production of TNF-α and IFN-γ cytokines. However, pre-exposure to LprN protein failed to provide protection in mice after challenge with a virulent strain of M. tuberculosis. Histological examination showed an increase in tissue destruction in experimental animals, indicating an immunogenic potential for LprN protein that enhanced the virulence of bacilli.