Insights from the Genome Sequence of Mycobacterium lepraemurium: Massive Gene Decay and Reductive Evolution

Genome downsizing and pseudogene formation.At 4.05 Mb, the M. lepraemurium genome is the smallest within the MAC and one of the smallest among mycobacteria (9th out of the 350 sequenced mycobacterial species). More strikingly, within the mycobacteria, only the M. leprae and M. lepromatosis genomes contain fewer functional protein-coding genes than the M. lepraemurium genome. The presence of 1,139 pseudogenes indicates that M. lepraemurium underwent reductive evolution, which is characteristic for strictly host-associated organisms. Analyses of pseudogene families within a diverse set of prokaryotes have shown that pseudogenes are most likely to occur in ABC transporter, short-chain dehydrogenase/reductase, sugar transporter, cytochrome P450, and proline-glutamate (PE)/proline-proline-glutamate (PPE) gene families (11). The M. lepraemurium genome was found to contain pseudogenes in all these families.

M. lepraemurium is the third mycobacterial species known to have undergone reductive evolution. The other species include the common ancestor of M. leprae and the closely related M. lepromatosis, which underwent genome reduction before the two species diverged, as well as Mycobacterium ulcerans, which is in a state of intermediate reductive genome evolution (12–14). Genome size among M. avium subspecies varies considerably (around 4.8 to 5.5 Mb), but no extensive pseudogenization was observed in these organisms. Curiously, M. lepraemurium seems to be evolving convergently toward a minimal gene set such as the one retained by M. leprae (Fig. 2). This may be due to both species having adapted to a similar niche, consequently resulting in similar pathological manifestations.

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FIG 2 

Heatmap of the gene orthology between M. avium subsp. paratuberculosis K-10, M. lepraemurium, M. leprae, and M. ulcerans. Genes are shown in black, and the absence of genes is shown in white. The raw height corresponds to the number of genes.

Loss of the DnaQ-mediated proofreading mechanism of DNA polymerase III α-subunit has been hypothesized as the cause of pseudogene formation in M. leprae (12), and a similar pattern has been observed in other obligate pathogens and symbionts (15, 16). In M. lepraemurium, a homologue of the error-prone repair DNA polymerase III α-subunit (MLM_3495) is nonfunctional, which might contribute to a higher error rate leading to pseudogene formation in this species.

The AT content of the pathogen/symbiont genome seems to correlate with the age of the strict association with the host (16). As expected, the genome of M. leprae has the highest AT content of all mycobacteria (42.2%), especially in pseudogenes (43.5%). On the other hand, the genome of M. lepraemurium is as AT-poor as that of its closest relative, M. avium (31%), and shows only a small difference in the AT content of functional genes and pseudogenes (30.46% and 31.05%, respectively). This suggests that M. lepraemurium adopted the “strictly intracellular” niche relatively recently, as evidenced by its phylogenetic relatedness to the free-living M. avium (Fig. S1). Genomic data for other M. lepraemurium strains would allow us to determine whether the pseudogene content varies between strains, which would indicate ongoing reductive evolution.

Repetitive sequences and mobile elements.Repeats and mobile elements are the major vehicles leading to genomic rearrangements and large deletions, and hence, they are thought to play an important role in genome downsizing. Yet, there seems to be a negative correlation between the stage of reductive evolution and repeat content (17). This is likely because deletions and pseudogenization impact fitness as more genes become nonfunctional, and the remaining genes become indispensable. Therefore, it has been suggested that repeats might be involved in the early phase of reductive evolution and that a different mechanism is responsible for the gradual deletion of pseudogenes (17). M. lepraemurium has only a few repeats (Table S1), which mostly derive from three families of dysfunctional insertion sequences (IS), each consisting of up to six copies.

Curiously, one IS family consisting of six copies (MLM_1414, MLM_1873, MLM_2691, MLM_2913, MLM_3078, and MLM_3876) shows 84% nucleotide identity with the ISMsm2 mobile element from Mycobacterium smegmatis (NCBI accession number WP_003887303). A BLAST search showed that this IS family resides in only a few other unrelated mycobacteria and not in the MAC, with the exception of a more diverged copy (with 78% identity) present in the plasmid of M. avium subsp. hominissuis strain 88Br (GenBank accession number KR997898.1 ) and a truncated copy (with 81% identity) in the genome of M. avium subsp. hominissuis strain H87 (GenBank accession number CP018363.1 ). Strikingly, when we analyzed the genomic synteny between M. lepraemurium, M. intracellulare, and M. avium subsp. hominissuis, all the synteny breaks in M. lepraemurium occurred at the locations of this particular IS (Fig. S3A and B). The synteny plot between M. lepraemurium and M. avium subsp. paratuberculosis looked different from the aforementioned genomes, which is probably due to genomic rearrangements in M. avium subsp. paratuberculosis (Fig. S3C).

The presence of identical copies of dysfunctional transposable elements indicates that M. lepraemurium lost functional transposases relatively recently. As mentioned above, at least one IS family was responsible for large genomic rearrangements in M. lepraemurium, and a deeper analysis of the genome sequence might shed light on the driving force that caused major deletions in this genome.

M. lepraemurium-specific sequences.Comparison of M. lepraemurium with other members of the MAC revealed a total of 32.2 kb of genomic sequence that was not found in any publicly available genome sequence. These regions contain 35 genes, of which only 12 are functional (Table S2). One of these genes, MLM_3300, codes for a Fic family protein. Fic proteins have been associated with pathogenicity in bacteria, often acting as toxins that interfere with the host cell in different ways (18).

Interaction with macrophages.After entering the macrophage, members of the MAC reside within phagosomes (19). There they inhibit phagosome maturation by preventing acidification to a pH below 6.4, and thus, prevent fusion with the extremely acidic lysosome. In M. avium, mutations in the PPE gene MAV_2928 and the PE gene MAV_1346 inhibit maturation and acidification of phagosomes, resulting in decreased virulence (20, 21). In M. lepraemurium, gene MLM_2357 (homologous to MAV_2928) and MLM_1265 (homologous to MAV_1346) are functional, suggesting that they may help its survival in macrophages. Additionally, the gene MLM_2012 encodes the lipoprotein, LppM, which is an important virulence factor in M. tuberculosis and interferes with phagosomal maturation in macrophages (22).

Reactive oxygen species (ROS) are produced by the macrophage as a bactericidal mechanism in response to infection. The ability of pathogens to produce enzymes such as catalase-peroxidase, epoxide hydrolase, and superoxide dismutase (SOD), which remove ROS, enable their survival within macrophages. While it has been suggested that M. lepraemurium abolishes the production of ROS upon entering the macrophage (29), probably as a survival mechanism, it remains unclear whether small quantities of ROS are produced upon infection and handled by the bacterium. M. lepraemurium has retained four functional epoxide hydrolases (MLM_0642, MLM_0684, MLM_1194, and MLM_1485) and one catalase-peroxidase (MLM_2092), which explains its observed catalase-peroxidase activity (23). Moreover, M. lepraemurium is able to produce two superoxide dismutases, SodA and SodC (MLM_0123 and MLM_2650).

Glycopeptidolipid synthesis.Glycopeptidolipids (GPLs) are synthesized by several nontuberculosis mycobacteria, including members of the MAC. GPLs are present on the outermost layer of the cell wall, and therefore, they play an important role in sliding motility, biofilm formation, and pathogenesis (24). This is even more relevant within the MAC, whose members can produce a number of serovar-specific variations of GPLs. The M. lepraemurium cell wall contains glycolipids and amino acids (25, 26); however, production of GPLs in M. lepraemurium has not been studied. Most of the genes known to be involved in the biosynthesis of non-serovar-specific GPLs are intact in M. lepraemurium, albeit with a few exceptions (Table S3). Although it is difficult at this moment to predict the effects of these mutations, it appears that M. lepraemurium should be able to produce GPLs, as all of the core genes are present.

Virulence.While the ESX-1 system is the main determinant of virulence in M. tuberculosis and M. leprae (27), it is absent in the MAC. However, ESX-5, which is also associated with virulence in pathogenic mycobacteria, is mostly intact in M. lepraemurium, except for the cytochrome P450 hydroxylase (MLM_2361), which is also nonfunctional in M. leprae. The duplicated four-gene region, ESX-5a, encoding EsxI, EsxJ, a PPE, and a PE protein may serve as an accessory system for transport of a subset of ESX-5 proteins (28) and is still functional in M. lepraemurium. In M. lepraemurium, the PPE and PE genes are merged into a single open reading frame, as in M. avium subsp. paratuberculosis but not in other members of the MAC. An interesting observation is that in MAC, esxI and esxJ are 100% identical to esxN and esxM from the main ESX-5 locus (unlike in M. tuberculosis), which indicates a novel and recent duplication event and suggests that a crucial function might lie behind redundancy of the ESX-5 components.

The pks12 gene is involved in the biosynthesis of mannosyl-β-1-phosphomycoketides (MPM) and is found only in the slow-growing mycobacteria. In different pathogenic mycobacteria, including M. avium (20), pks12 was shown to be necessary for the virulence, and this gene is functional in M. lepraemurium (MLM_2156).

Experimental procedures. M. lepraemurium strain Hawaii was grown in BALB/c mice. The DNA was sequenced using Illumina and PacBio technologies, followed by sequence assembly and annotation. More details are given in Text S1 in the supplemental material.

Accession number(s).The annotated genome was submitted to GenBank under GenBank accession number CP021238 .